Often used in chats of multiplayer racing games and presumedly derived from the Need for Speed series, "gr" is an abbreviation for "good race".
It is used in praise of your opponent(s), usually after a hard and fair fight for positions.
Today, "gr" isn't that popular anymore. Might have to do with the fact that racing game publishers concentrate more on the "bling bling" factor than proper online netcode.
GRS stands for Gender Reassignment Surgery. It is medical process whereby genitals are transmogrified into those of the opposite sex. This acronym is in common use in transgender community.
