Gatekeepers are typically labelled as fighters young prospects step up against to get to solid contender or title contention status.
Guys like Cotto are not close to being gatekeepers yet. Pavlik isn't either. They only fight other top champions, big names or number 1 guys in their division (unless they are taking tune ups after a layoff like Pavlik). That is so far removed from what a gatekeeper, that it's just absurd to call them gatekeepers.
They might get a title shot here and there, but they're generally beyond title challenges. They are used for young guys trying to pass their first big test, hence gatekeeper. They have enough of a name left for tune ups against champs etc still, but are basically used as fodder to test rising guys on.
Someone mentioned Miranda, and he is a great example. He's no threat to the champs, got a decent name and still can be risky so he's used to for young guys coming up. Bailey is a good gatekeeper, and has shown himself to be risky still by beating one of the young guys.
Even Judah isn't really a gatekeeper yet. He's on the verge, and has been for some time. Not yet though.
Most of the guys mentioned in this thread are way above gatekeeper status still. They are the top guys in the division fighting in title fights etc!
Even Mayorga isn't really a gatekeeper. He's only fought Cotto, Mosley, Vargas, Oscar and one journeyman who he beat in the last five years. That's not what a gatekeeper does.