There is no general answer. Each Gemach is different, but nowadays I believe many sell clothing in really bad condition to third world countries by the pound. (It’s probably what the “other fabrics” used in stuffing etc. which gives us shatnez nightmares is.)
So you need to ask what their criteria is. Some so called Gemachs are really businesses — or so it seems. You might not care if you are dumping real junk, as you are giving a Yid parnassah, but feel free to ask where the money — as that is what it boils down to — goes.