Invented by a Blizzard moderator on the Warcraft III forum. There is always much whining going on on those official forums, much sucking up to moderators and an awful lot of 'BLUE!!! PLZ REPLY!!!1one' (Blue = color of moderator posts), so when a mod posts something, you can bet there'll be thousands of people jumping on it, if only to spam 'BLUE FIX GARGZ PLZZZZ' or 'first reply woooot!' after it in the vain hope that the mod will read it and react.
 
So when one invented this new buzzword, it was grabbed and squeezed out and spread like a virus across all of the Blizzard forums and from there over the internet.
 
The word is derived from a unit in Warcraft III, the gyrocopter (flying machine). Your immediate response when someone masses gyros (to be funny, ruin the game, mess around or whatever) is to rofl for ten straight minutes until the copters of doom and destruction have pinpricked 1 of your 20 farms to death with their uber godly bombs.
 
Now mostly used in Blizzard circles as a leeter version of rofl.