Here is a good response about what the gsm number means.
Ok what does gsm mean?
You don't see this as commonly as you see a shirt listed as 20/s, 30/s, 40/s, or 50/s in reference to it's thickness. The GSM has to do with a shirt's weight, which, of course, is directly related to its thickness. A shirt's thickness is related to the weight (or thickness) of thread that is used to weave it. A 50/s (or 50 single) shirt is a shirt that is woven with a 50 single needle yarn-- a very fine yarn. This makes a very thin, fine, soft shirt because a tighter and finer weave is able to be achieved. An 18/s (or 18 single) shirt is a shirt that is woven with a 18 single needle yarn-- a thicker or coarser yarn. This makes a thicker or coarser shirt.
I believe this is what you are asking about. Your 115 gsm shirt is probably a 40/s shirt. Possibly a 50/s shirt, but those are not as common.
Oh, and in terms of the GSM, the lower the gsm number, the thinner the shirt would be-- I believe that is also what you were asking?