GP is a kind of virtual currency here on E2, which can be used to purchase things at the E2 Gift Shop and similar nodes.
GP can be earned by voting (1/3 chance per vote), being blessed by an administrator, participating in quests and contests, getting egged by other users in the chatterbox, and possibly other ways as well.
Unlike XP, which stands for "experience points", nobody seems to know what "GP" stands for, if anything at all. But suggestions have included "goodness points," "gravitas points," and "grapefruit pie".
Users who don't want to use GP can opt out of it in their User Settings.