the only surefire way of telling is indeed the GT stamp at the end of the top tube.
However, that triple triangle, though it looks very similar to the GT one, seems to have a lug join on the seat tube immediately below the top tube. GT have always to my knowledge 'pierced' the toptube with the seat tube.
I've never known GT to do internal cable routing.
I've seen one eyelet but not 2, maybe some of the lower end bikes did have them?
Other bikes did have a triple triangle style design, maybe Townsend but this was not a 'pierced top tube design, just a butted joint.
We need more info from OP.
Although I have to admit as a Yank I wasn't familiar with all those other triple triangle designs (except some Japanese ones), I will say my '93 Outpost has double eyelets at the rear triangle like that, and my girlfriends '96 GT Pantera doesn't have the "GT" stamped into the back of the top tube where it protrudes behind the seat (which surprised me when I picked it up for her). So I have to say that isn't
a surefire way to determine whether it is a GT or not.
Note this is what a stock '89 Karakorum looks like: