A entire game being more difficult doesn't really impact whether GMing is more difficult. You have to make the specific case. And, from what I've seen in this thread, the reasons cited for GMing being more difficult in playing are preferences, not requirements. It's also very D&D specific, which ensaddens me.
As for gatekeeping, I do see the reasons given in this thread for why GMing is hard as the kind of gatekeeping that accretes around an activity. There's a preponderance of opinion that GMing requires being good at this host of things that are, ultimately nothing but preference and tradition. And, that gestalt conception of what GMing is (at least in D&D) erects a barrier to entry. If a new GM uses the random dungeon creator in the DMG, puts in some monsters and a trap, and runs an Orc and Pie game, it's indisputable that they are GMing D&D, maybe well for the goals they have. But, according to this thread, they haven't yet reached the bar for GMing because they aren't acting out NPCs, they aren't taking copious notes, they're scope isn't big enough... pick a post from this thread and you'll likely find some added requirement for GMing. That accretion of traditionals and expectations is definitely gatekeeping. Is it intentional? No, I don't think so, in that people don't intend their ideas to be gatekeeping, but it does that anyway. Someone that wanted to start GMing that read this thread will come away with the idea that they have soooo very much they have to do, and that doing it is hard, and that's gatekeeping.