GTS usually means go to sleep over text. But GTS has lot of meaning it has different meanings in different categories and in different perspective.
GTS according to the urban dictionary it stands for go to sleep. There people who use GTS for good times.
GTS can be also stand for global trading system. So GTS meaning defers from category to category. But most of the sources states that GTS stands for go to sleep. Other details around the term GTS or goody to shoes, ghost in the shell, Google that stuff et cetera.