Gainer is slang for the diving trick reverse. The category number is 300. For Example, a reverse flip (usually the term for gainer) is 302, and reverse double would be 304. A reverve is done when walking forward with a full approach, and not running. Many inexperienced divers run on the board, and could very easily slip. Gainer 2 and a half, 305, pike is the dive Greg Lougains hit his head on in the Olympics.
My personal favorite dive is a 304 tuck, which is a reverse double, but I like to call it, "gainer double."