Gangbang - Group sex wherein multiple people (typically with penises) have sex with the same individual (typically one that does not have a penis).
Gen - Short for "generous" this refers to an individual looking to pay money in exchange for a sexual encounter.
G.H. - Gloryhole
G.I.L.F. - Grandma/Grandpa I'd Like (to) Fuck
Gloryhole - A hole in a wall (or sheet) typically intended for an anon to stick their penis into for the purposes of recieving pleasure from another individual. Typically this involves oral sex, though it is possible for this to refer to other activities involving pleasure of the penis, such as anal sex, vaginal sex, or a handjob.
Golden Shower - When one (or more) individual(s) urinates on another individual(s).
Group Sex - Sex involving three or more people.