I have a 2005 Chevy trailblazer, I put a new radio in it because the previous owner took it out, and overnight it drained the battery so I jumped it, and now the fuel gauge has been showing up almost all the way down at the bottom in the middle of the gas gauge, and I’m not sure what to do about it as it’s never happened to me before. My gas light still works, I checked fuses just to see if maybe that was the issue, but no fuse was blown. The car starts right up and everything but I’m not sure what to do about the gas gauge.

 

try unplugging the stereo and see what happens.
Or maybe you disturbed some wiring in the dash while installing it. knocked a connector or ground loose.
alternatively the drained battery + jump start might have cause a glitch. Try a reset by unhooking the battery and holding the brake down.

 

It could also be a bad stepper motor. Stepper motors going bad is not an uncommon occurrence in the TrailBlazer/Envoy/etc. platform.

Also, one has to make sure that the wiring harness used for non-factory stereo/audio systems is compatible with the vehicle because most everything is controlled by the Body Control Module.

If the radio you are installing is an aftermarket radio, then you really need to visit crutchfield.com to see what radios are compatible and then either purchase the radio and installation kit from them, or risk doing real damage to the vehicle's computer systems.