And getting those trinkets on mythic is the reward for doing so. It's not "gatekeeping" it's the last true advantage having to actually socialize in an MMO has.
And I would disagree: Mechanically a +17 isn't different than a +12. Mythic raids on the other hand are different than their heroic counterparts. And raid fights are mechanically much harder than m+. So just throwing 20 players that can clear a +17 together would not be enough to clear mythic Gallywix.