Had a look on here - GT serial cheat sheet.
http://bmxmuseum.com/forums/viewtopic.php?id=71652&p=2"Under Serial number will be a model #: JR = Junior, 3=Expert (17.5" TT), 4 =PRO 18.5" Standard - measure top tube, if longer it is an ordered XL, - 24/26=Cruiser(Not sure of Letter Code?)"
then someone posted this
"3=Expert is incorrect. The expert frames are designated with the number 2. This goes for 81-82 and 83 to Early 84's. Some 24inch cruisers for these years were stamped 24, and others were stamped 4. The GT 26's, as far as I am aware, weren't stamped with a model number. GT did make a junior 24 in late 82 through 83/84, and I believe (but it hasn't been confirmed) that these were stamped with a 2 (being that they were an "Expert" sized cruiser.). These junior cruisers were built in VERY limited numbers. I've yet to see one, but I have heard a bit about them.
I have a 1981 GT Junior frameset (stamped JR), a 1983 GT Expert (stamped 2), a 1983 GT Pro (stamped 4), a 1982 GT 24 (stamped 4) and a 1981 GT 26 (not stamped with a model number)"
Edit: Just checked a f&f I have and it's a GT Expert 1983 - stamped 2