The origin of the expression was, I believe, that waking up with glass in your mouth was irrefutable proof that you were insane: one who does this has "the GIMS". In its current usage, an attack of the GIMS can range from something as trivial as excessive worry or brooding all the way up to a psychiatric admission to hospital.
It is perfectly acceptable (at least, in my circles) to use "Sheet, bro, I got teh GIMS" as a reason (excuse?) for, say, not getting dressed for a week, not going to work, screaming at small children until they cry, or indeed screaming at small children until they cry while undressed and staying home from work.
If a person or thing gives you GIMS, then he/she/it is termed "GIMSeous"
Disclaimer: The purpose of this writeup is to expand upon Fionacat's definition of GIMS. Some of you might find it offensive or somehow feel that it pokes fun at the mentally ill. The source of this confusion is possibly due to the use of gallows humour and/or sarcasm. Why not try getting a more intelligent friend to read the writeup to you and listen to the inflection in his or her voice, or better still just sit in the corner with your hands on your knees?