G.O.A.T. means Greatest Of All Time.
It's a general superlative, and not specific to gaming, but IME you'll see it most commonly in marketing for games or music. Dictionary.com suggests it's also often used for athletes.
RPG stands simply for “role playing game”. There are a few factors that classify RPGs
as such. Roleplaying, of course is key here – you take up the mantle of a fictional
character. The best RPGs will involve you traversing a fully fleshed out world to become
MMORPG stands for “massive multiplayer online role playing game”. “Massive