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.