It looks like an early GT. Looptail was common then. I'm that that knowledgeable on BMX, perhaps others can add more details.

Interesting to note that at this point in their frame design the seat stays end at the seat tube rather than the more forward to land on the top tube. It also looks like it has a derailleurs front and rear. Hmm.

Early GT's and BMX 26" wheel bikes had either a coaster brake or single speed freewheel, not derailleurs. The brakes and headset look lower end. GT's during the BMX period had better quality components. I am wondering if this was a bike from a period when the GT name was sold and the new owner many have cashed in on the GT name with a more affordable bike.

It does have the GT top tube that extends beyond the seat tube. I'm hoping someone else has better answers than my lame observations.