Is a pipelauncher actually a wonderous antigravity device?

A pipelauncher is certainly a machine that requires gravity is order to function. There are few machines that actually require gravity to function. It is true that your car could not function in zero gee, but this more a case of it being designed to function in a gravity environment- one could modified it so that it functions in zero gee. A pipelauncher simply wouldn't work in zero gee- modifing it is not possible. Another type of machine that won't work in a zero gee are things like hot air balloons or helium balloons.
Since balloons can be called lighter than air flying machines, you could instead call a pipelauncher a lighter than water machine, but a since pipelauncher is dependent upon gravity and increases in it's power if one were to increase the force of gravity, it's not really much of a stretch to call it an antigravity machine.

What is a pipelauncher? It's a simple machine. No, it not one of the six:
"You are probably familiar with many different machines. Some of these machines appear highly complex. However, all machines, no matter how complex, are made up of one or more of the six simple machines. The six simple machines are:

1. Lever
2. Wheel and Axle
3. Pulley
4. Inclined Plane
5. Wedge
6. Screw "
http://www.uark.edu/depts/aeedhp/agscience/simpmach.htm

But it *should* classified as a simple machine. The pipelauncher doesn't need any of the above 6 simple machines. A pipelauncher is essentially a pipe with one end capped. The purpose of a pipelauncher is to lift rockets. To function as a mobile launch pad for rockets from water.

An ideal pipelauncher could be a km or more in length. In the near term, one could say that a pipelauncher would be limited to adding sub-sonic speed to a rocket. But in practical terms it would more reasonable to start with much smaller pipelaunchers and also much slower speeds than +500 mph. If a pipelauncher is say less than 300 meters in length, than one doesn't need expensive material and overly complicated structure. The shorter length limits the amount speed that can imparted to a rocket, but the main value of a pipelauncher could be a way to make a very cheap launch pad with any added speed as a bonus.
I would recomend making a pipelauncher from dozen or so large pipes [2-6' in diameter] which would welded together to form a larger pipe. But I use a simpler shape in order to quickly describe it's characteristics.

Take a pipe 200 meters long, 20 meters in diameter and pipe wall thickness of 2 cm. Use a high strength steel which can be easily welded.
Since steel has density of 7.8 grams per cc or 7.8 tonnes per cubic meter, one can determine how much this weighs and it's displacement in water:
Circumference = diameter times Pi, so roughly this pipe is equal to 62.832 meters by 200 meters which equal to 251.328 cubic meters of steel, times 7.8 equals 1960.36 metric tonnes.

If both ends were capped, this pipe would displace:
Pi times 10 squared times 200 = 62,831.8 cubic meters.
This means a few things.
If you added 60,871.44 cubic meter of water inside the capped pipe
the pipe would be neutral buoyant in water. Or other way of saying this
is that if you applied 60,871.44 metric tonnes of force you could push this capped pipe under water. Or another aspect is that the weight of steel being 1960.36 metric tonnes is about 1/32 of weight needed to make capped pipe neutral buoyant.

This also means that if were to lower this both end capped pipe vertically in the water, that 1/32rd the length of the 200 meter pipe would sink before the pipe began to float [200 divided 32 is 6.25 meters of pipe would submerge].
But of course a pipelauncher is actually a pipe with just one end capped. What happens if you were to lower this pipe with only end capped so that the open end of pipe was put into the water?
The answer is the pipe would sink until such time that the air inside the pipe reached a pressure somewhere around the same pressure required to depress the water inside the pipe 6.25 meter below sea level.

Sea level air pressure equals 14.7 psi and this is 1 atm. Each 33' or about 10 meters of ocean depth is equal to 1 atm of pressure [14.7 psi]. So, .625 times 14.7 equals 9.1875 psi. The pressure in the pipe would need to increase by 9.1875 psi in order for the capped pipe to float.
If you half the volume of a gas, you double it's pressure. Roughly speaking if you lower a 200 meter long pipe with one end capped 100 meter under the water the pressure inside with will be 2 atm- twice the pressure of sea level pressure. But it is less than 1 atm of pressure- it's .625 of 1 atm. The answer is that pipe would need to lowered *about* 62.5 meters before it floated. [this isn't exactly right, because the section of pipe under water weighs less- steel would weigh 6.8 metric tonnes per cubic meter under water instead of 7.8 metric tonnes. Additionally you would measure from point where water level inside pipe is depressed below the sea level {62.5 meters plus 6.25 meters = the end of pipe would be 68.75 meter below sea level} but all this somewhat uneccessary detail].

Anyways, with the pipe with one end capped which is only about 62.5 meters below the surface with 137.5 meters sticking above waterline, it probably be somewhat unstable- in terms staying vertical. Besides having the top end over 30 stories above the sea makes rather difficult to put a rocket on top of it:).
By bleeding air out of the pipelauncher you could lower it- you could instead try to pull it down- but this would require a tremendous amount of weight [1960.36 metric tonnes would lowers the water level inside the pipe by 6.25 meters, 19,603.6 tonnes lowers it by 62.5 meters, etc]. If you bleed the air out of pipelauncher it will come out at 9.1875 psi- if you add air into pipelauncher at more than 9.1875 psi the pipelauncher will go up.

Ah, that reminds me, let's change the scenery. Let's go on the land. Let's put this 200 meter long pipe so it's standing vertical on land. It weighs 1960 metric tonnes or 1,960,360 kg. With circumference of 62.832 meters [6283.2 cm] and pipe wall thickness of 2 cm that's total area of 12,566.4 sq cm giving 156 kg per sq cm. That would easily dig into dirt, but concrete could easily withstand that. Now add 5 psi to inside pipe pressure. This will reduce weight on the 12,566.4 sq cm of pipe wall.
Now add more air pressure so it's 9.1875 psi, and this 1960 metric tonnes thing would hover- as long as you can add enough air so it maintains the 9.1875 psi inside the pipe. Of course when I say hover, I'm talking about less than say 1 mm above concrete, it would take a huge amount power to hover say an inch above concrete.

Back to ocean. Lower pipelauncher so it's say the top is 20 meter above sea level. If you were to put 1960 metric tonnes of rocket on top of pipelaucher it would lower it down by around 10 meters and increase the pressure inside pipe to 18.375 psi with water inside depressed 13 meters below sea level. In order to accelerate up say to 1 gee, with this 1960 metric tonnes of rocket payload [btw, current biggest rockets are about half that mass, nor could rocket handle as much as 1 gee], you need to add enough air pressure inside pipe so water level inside pipe depressed twice 13 meters- 26 meters.
The way you would do this is by using both heat and cold.
The cold would in the form of liquid air. And the heat can be kerosene or whatever fuel you want.
You could slowly pump the pipelauncher back up to say, the top is 50 meters above sea level, then add a little bit fuel so the air is heated up inside causing pipe to rapidly rises to say 80 meter high, and then dump in liquid air to cool the air, causing it to drop down near sea level, then dump in lots fuel and at end of acceleration phase, launch the rocket.

This doesn't require much liquid air or fuel. Nor does require that you get the air very hot- the range might something like -150 C to 600 C and the extreme ends of these temperatures would last less than 10 seconds. Therefore the structure need not get very cold nor hot.