Re:
6061 is what GT used for their top spec MTB's, like the Zaskar, so that gives a clue to where it was in the range.
I don't know much about GT road bikes, but take a look ion the inside of the non driveside dropout. You may find a frame number, and if it follows the same code as the Zaskars (which were all hand welded) then the first two digits will be the year and the next two month of manufacture (or it could be the other way around.... it's been a while).
Do some digging on GT road frames, as they built a fleet of frames for an American pro team who used them in Europe including (I think) the TdF. This is all ancient stuff I am dragging from deep inside my often faulty memory, but I remember searching for one of those frames.... and not finding one.
Enjoy your GT.