How do you register gun if do not have receipt?

Depends entirely on the law where you live- and you did not share that with us. In the US, there is NO registration of ordinary firearms on a Federal level. A few states do require registration, most do not. We also get questions from several nations outside the US- and THEIR procedures are different. Please repost giving the jurisdiction where you live, and we can give you a better answer. You can also check with gun dealers or law enforcement in the area where you live. Sorry-