Every time I turn the car on, it gives me different gas readings. Sometimes I fill it up, but the gauge reads empty or halfway—and vice versa. The check engine light is on, and I know it’s because of this. I had the codes read, and it gave us a specific code, but I honestly forgot what it was. I just know the light is on because of the gas reading issue. The problem is that I don't really know much about how much the whole thing is gonna cost. Also I turned on my car the other day and it started revving by itself.. Is it GGs?

 

The incorect gas readings are becoming a common issue with these cars. The problem is often traced to a faulty solder joint on the circuit board within the UMAC amp. This thread has a video which shows how to replace the amp. As for replacements, I would just scour eBay or any local pull-a-part junkyards you may have in your area and get one from there. Unless you are skilled in repairing circuit boards in which case you can fix it yourself like a fellow forum member did here.

Without codes we have no idea what is going on with your car. The engine "revving by itself" may or may not be related to the gas level issue (doubtful, IMO). Check for codes again, report the results here and go from there.

 

When I hear cars revving by themselves, I'd speculate a problem with the throttle-by-wire, sender or receiver?

 

"Assumption is the mother of all foul-ups." Without a proper description of symptoms and/or a thorough diagnostic there is no way to know what behaviour the car is exhibiting. Let alone how to fix it.

If I were to guess, the "engine revving by itself" is idle surge. A surging idle can be caused by a litany of things (bad vacuum hoses, dirty throttle body's, fuel injector(s), emissions sensor issues, etc.) but I would highly doubt there is a issue with the "drive-by-wire" electrics themselves. Possible but not probable especially with these engines.

Again, OP, take some initiative: have your car checked for DTC's and post them here rather than assume it is the fuel level or otherwise.