Someone tell me what OP and GD stand for lol

When used in the context of the league forums OP can mean "Original Poster" or "Original Post" and GD usually means "General Discussion" which is the most active sub-forum in the older vBulletin format forum, the one you're using right now was previously known as the "Community Beta" or CB, and is now called the "Boards". You'll probably still see it called CB in a lot of places

op is overpowered. gd is maybe game devs... but im not sure on that. Alos the "trolls" tht say op doesnt mean overpowered are in fact trolls

Along with what Sir ArmaMalum said, here are a couple of other ones:

tl;dr -- too long, didn't read. This is used when someone wrote a long post and wants to give a summary.

ITT - in this thread