Sorry to hear that but do keep your hopes up. Small fish like that often tend to go "missing" for a while and come back out of nowhere. Check your overflow box/chamber and your sump for it, also check around the tank as they are jumping fish.
do you see them eat at the fish store before you buy them? Are their bellies full or sunken when you buy them? Maybe you have other fish that won't let newcomers join?
No he said it was there for four weeks and didnt lk slim and my fish dont care really if any newcomers come in but i have seen a big hole at the front enough to fit a goby in but at the shop he wasnt very active and i thought thats what they did i hope hes not dead but if he is i will never add another goby in my tank again