Mars is a "gateway" because it can provide a wide variety in-situ resources, has a gravity strong enough to sustain human physiology but low enough to make escape easy. Mars has all of the basic chemical elements necessary for terrestrial life along with abundant water as ice. Mars also has an atmosphere that is useful for resources and aerobraking.

The Moon does not have an atmosphere and lacks most volatiles (got cooked off when it was formed). The Moon's in-situ resources are insignificant (don't buy into the He-3 flim-flam and water nonsense). We'll invest resources on the Moon until the public grows tired of the process (like the ISS) and then withdraw with nothing to show for it (a repeat of the Apollo program). The Moon has already demonstrated that it is a dead-end. We're about to have that very same lesson repeated.

Gary