Sometimes, people misunderstand one another because they use different words to describe the same thing or the same word to describe different things.

To me, a space "gateway" is a location for providing the full range of logistics support to spacecraft. In that usage, I see LEO as being the best place for a gateway. Long ago, Heinlein said that once you're in Earth orbit, you're "halfway to anywhere." I still believe that to be true. In military studies, amateurs talk about tactics while professionals talk about logistics. It takes a lot of logistics support to mount long range space missions, especially manned ones. The logistics required includes trained crews, propellants and other consumables, systems manufacturing and repair capabilities, as well as long range navigation and communications capabilities.

Mars would make a lousy gateway, IMO. It'll be generations before we have the ability to manufacture the components needed to build and service spacecraft between missions, to train the crews, and to launch new systems. It may be possible to refuel spacecraft at Mars, but if you're planning on landing in order to refuel, you're adding a lot of complexity and mass to the vehicles. If you want to refuel in orbit around Mars, then you have to first slow the vehicles to put them into orbit and to launch the fuel up there to meet it. Neither is an overly difficult task but not trivial, either.