Check your fuses.
The GMC dash tail and parking lights are on a different fuse from the brake lights. Check the fuses for a possible blown fuse.
It could be a blown fuse. Tail lights and brake/turn are on separate fuses. Apparently there is not a ground problem since brake/turn works OK.
check the fuses
Brake light switch ?
check your fuses
on some cars the tail lights and brake lights are totally different bulbs gain access to the bulbs and have someone push the brakes so that you can determine which bulbs are the brake lights and replace the tail lights
There are no extra fuses for your trailer lights. If you have the towing package (and you do) the trailer lights run through your regular fuses (tail, brake, turn).
Check fuses or headlight switch
Check the fuses first then check the bulbs.
98 sierra1500 4x4 all lights work excepttail lights fuses are good bulbs are new
Check fuses, common circuit