The informal name applied to the status of a boxer who is well skilled, high ranking and who would provide a test for another fighter who aims to become a contender. They are higher status than a journeyman, so the prestige of beating one is greater, but they are not currently themselves a likely title contender.
Former contenders or champions, who are older or no longer at the highest level may be referred to as gatekeepers for their weight class. Meaning they ‘gate’ the championship contention, especially for the top prospects to prove themselves as they rise through the ranks.
A boxer may not like to be referred to as a gatekeeper (and similarly journeyman), but fans and commentators will use these terms and debate who falls into which category.
Boxing Unboxed 