Except for SLI the acronyms are all made up for marketing hype. Also it is diffrent depending on model number.
X = Extreme
GT = Grand Tourismo ... derived from a race class for street cars and used there
GS = Grand Sport, class below Grand Tourismo
XT = More Extreme than extreme
GTX = Grand Tourismo Extreme
GTO= Famous Car
GT, GS, GTX, GTO come from cars and are associated with good performance by a lot of people, thats why they are used.
X, XT, XxxxXT = X is used for eXtreme since 10 years now and recognized as that.
They dont mean anything for the card... and sometimes the cards with extra letters is worse than the one without any letters, as well as there were manufacturers selling the weaker chip in a card labled like the better ones and overclocked... So a GT card with GS chip overclocked and running hotter& less stable.