Yep pretty much all the triple triangle GTs used U-brakes on the rear due to heel clearance problems with wide profile cantis up until 1992.
1993 saw GT ditch the U-brake because the newer low-profile cantis solved the heel clearance issue.
So assuming the bike in question has U-brake mounts, which it appears to have, then that dates it no later than 92, and as we know the 1992 Pantera also had a flat end cap on the top tube.
So yes it's 50:50 Zaskar/Pantera.
I'd hack off that ugly disc mount, build it up and call it a Paskera.