When Gary's sleeves are pulled down he has on a hospital bracelet and what appears to be either long sleeves or bandages. Is this suggesting he attempted suicide or was he simply in the hospital for drug and alcohol abuse?
I think he did try to attempt suicide. He had bandages on his arms where he had cut himself, and also notice that his coat sleeves had loops so that he could pull them up over his thumbs, so they wouldn't slip down and show the bandages. You see this when he says "We've got ink on out hands!"
Also when they're in bowls clubhouse Andy demands that Gary show them his arm to prove it's him and Gary outright refuses, he obviously doesn't want anyone to see his bandages.
This film is quite dark and serious in a way, certainly with Gary, at least he has a happy ending...sort of!
I understood that he'd slashed his wrists as well. When he initially refused to show his arms I overthought it and assumed there were track marks he didn't want anyone to see but I was wrong.