Yep, its a GT RTS-2. I'm the original owner from the mid-90's. It's got a couple vintage aftermarket parts like Onza H.O. brakes, Pulstar hubs (purple!), WTB dual compound brake pads, and Mag 21 fork. I rebuilt the fork and re-cabled it last fall after I cleaned out the fossilized Shimano grease from the shifters.
I pulled the crank and lo and behold there is a similar crack (although smaller) in the same place on the drive side. Even if I can get it welded up, I would probably retire it to leisurely road duty.
If you push hard enough, you might be able to get a warranty frame out of GT for cheap. I just got done doing that with my old avalanche and you saw my new zaskar at the ride last night. Got it stupid cheap...