on further reflection...
I remember old GT frames used to have U-Brakes.
...belay my last comments on the option of a rollercam or IRD rotary brake. Because of the boss location directly behind the seat tube, you're pretty much limited to a U style brake, shitmano's, diacompe's or others. You'd have to drill a hole thru the seat tube to use a rollercam or rotary brake, since these don't use a straddle cable that can loop around the seat tube. For those you'd need a way to give the cable a straight shot at the center of the brake, thus the need for a hole.
When you set up the U brake, here's the tip: Instead of just looping the cable around the seat tube, which results in an overly long straddle and one that doesn't pull the U brake arms at an efficient angle; what you need to do is cross the straddle on the seat tube so, going from the straddle hanger, the straddle forms an X just behind the seat tube, so the right side of the straddle as it leaves the hanger ends up pulling on the left brake arm and vicey versey. This makes the "effective" straddle length much shorter, and pulls the brake arms at a more efficient direct angle, rather than pulling on them straight forward. GT used to have a little nylon bolt-on guide on the seat tube to help guide the straddle around it as it crossed. Lacking that, just cross them anyway, the straddle ain't gonna cut thru the ti seat tube.