2009 GMC Seirra occasionally fails to shift out of 1st gear.
I’m guessing your automatic transmission shifts are controlled by a computer, electronics activating solenoids in other words. So, most likely, there’s a problem with one of the sensors which the computer uses to determine when it is time to shift, or the sensor is ok, computer is trying to actuate a shift, but there’s a problem with one of the solenoids.
The main input to the shift algorithm is vehicle speed, so a good start would be to ask your shop to make sure there’s nothing wrong with the vehicle speed sensor. I expect however the problem is a faulty solenoid. For the whole scheme to work however, the transmission fluid has to be in good condition, at the correct level, and the transmission seals to be sealing. If sensors and solenoids seem ok, you could try replacing the old transmission fluid with fresh stuff. That will contain new seal conditioners.