To see if the brakes are binding, just turn it upside down and see if the brake discs wiggle freely.

If it drives fine, odds are the trans is fine, but it's just a matter of taking everything apart and see what turns and what doesn't. I would just take the trans right out and check the rear diff, and if it turns then inspect the trans.

Also, if you are just bashing, I would open up the center diff (inside the transmission) and replace the gears with the solid locker that should have come with the GST. This ensures that there is power to the front and rear wheels at all times. It won't turn as smoothly, but it will have more traction.

Refer to the CEN diagrams while you are ripping it apart and get familiar with all of the parts.

Don't worry, once you've disassembled and reassembled it a half dozen times, you will understand how it works entirely.

BTW, The adjustment for second gear is 5 full turns out
Third gear is 7 to 7.5 full turns out

look at the CEN diagrams for the 3rd gear clutch. There are two "Set screws" that you musnt touch, then there are the two adjustment screws for the 3rd gear shift. Don't turn more than 1/8 tun at a time and eventually you will get it shifting.

But I would really worry about getting it tuned first, and making sure nothing is binding. THen once it's going good, get the third gear adjusted.