Ok, seat post first, a 1992 gt will take 22.2mm post. That is often referred to as a 1" post as the seat tube is 1" outside diameter.
If it has that sized seat tube/seat post, then it will need a 1" threaded headset. 1 and 1/4" is not used for bmx so either the headset you got is " not what it says on the tin " or you've not used enough force to tw4t the cups in!!