Gating is the restriction of content in a game to players/characters who've done particular previous content.
Early in MMOs, you had to be flagged, or have a key to get into some zones. By controlling access to the keys, a guild could lock down access to raid bosses that dropped the good stuff, for example.
In the current generation of MMOs, gating comes less formally than keys and flags. For example, none of my characters can do I.D. because they lack enough hit/focus to be of use. However, if a group with strong other characters wanted to bring me, I could come.
So the gating in Rift is done by level initially, and then gear.
My main hasn't had a meaningful upgrade in gear drop since I came to Gaiscioch. I don't see that changing until the expansion. I can grind out 120 infernal marks at 2.5 marks per day doing experts, and the 2 weekly quests, or I can give up on getting that gear. So... I'll see you when the expansion comes out, eh. -- that's a consequence of the gate principle.