Brake lights on my 01 gmc sierra truck dont work but the tail lights, blinkers and emergency lights all work.

There is a tail lights fuse for the truck, but not for the trailer.
First, check that your left tail light on the truck is working, if not fix the problem (new bulb, bad contact, fuse).
Then the obvious suggestions regarding the trailer are: Check the tail light portion of the bulb to be sure it is not blown. If good (best to put in new one to be sure), then check the wiring from the front of the trailer all of the way to the non working bulb, including every connection and at any place it is secured to the trailer frame. If all is good with the trailer circuit and bulb, then check the truck's trailer connector, including any adapters. If by chance the truck is wired with a flat 4 wire harness the problem may be solved (but usually the blinkers are the problem). Modern trucks require a 5 wire harness which self adapts to the standard 4 wire trailer circuit. Check for dc power (12 volts) from the truck's female connector. One wire will be ground, others will be taillights (turn on headlights), brake lights (have someone press brake pedal), and blinkers (turn on switch and blinkers).

First, verify that the brake lights work correctly with the old lights. If your blinkers work, the brake lights should work as well.
It is possible that your brake lights were not working previously. If they do not work, suspect a blown fuse or a defective brake light switch.

sounds like your switch on the brake pedal is bad or the wire between the pedal and the lights

replace the bulbs in the tail light, they are 2 stage, one for parking light and one for brake/signal

Most likely the rear tail light assys.

yep bad ground in the dash

The Blinkers and the Brake Lights are the same bulb. So check the brake light switch. Flashers use a different flasher unit, than the turn signals

most like u have a blown fuse most of the time in cars they connect the tail light and instrament lights to 1 fuse

brake lights and turn signals work off the same curcit, it could be the fuse or the brake light switch !!!!!!

Look up under dash where brake pedal pivots, you should see a switch that is activated by pedal arm.