The P0455 is "EVAP System Large Leak Detected". Very commonly this is a faulty Fuel Cap. Above you stated you replaced the fuel cap- be sure the fuel cap with which you replaced is as close to OEM specs as you can (the fuel caps sold at aftermarket stores like "AutoZone" do not meet OEM specs, and will NOT solve a P0455). If you positive the fuel cap is good, next check the fuel filler neck for any signs of damage or debris that will prevent the cap from sealing correctly. If thius check ok, you can next check around the fuel tank area for any signs of damage and/or broken vacuum lines. If the visual ispection checks ok.then the next best thing to do is to have the EVAP system "smoked". This is where a machine forces smoke through the EVAP system, and the "large leak" will reveal itself with a stream of smoke. Most shops charge around $45-$65 for this service, but it's money well spent, as chasing a leak in the EVAP system can be hard to do, especially if the leak is in a place where you cannot visually see, like say the top of the fuel tank.
Here's the diagnostic flow chart provided by GM for the P0455. (Notice the step that says to use a smoke machine. Might be the best bet! The P0455 isn't one of those codes where you can say "Oh, it's this....")
This sounds like a faulty Fuel Level Sensor. This is an extemely common fail item in GM vehicles. GM says it's due to the additives in the fuel pre-maturely wearing out the "teeth" that hit the striker on the sensor. New, updated versions are available. However, when this sensor is faulty, it will not set a P0455, it will set a P0462 or P0463 (commonly)
In my State, I am an Inspector- and to be perfectly honest with you... If the fuel gauge does NOT work correctly... SSSHHHHHH!!! Keep that to yourself. If there is NO p-code setting the engine light that SAYS the fuel gauge doesn't work (P0462) then who's to know the wiser?!?! If the code IS present, yes, it will need to be replaced. It's mounted to the side of the fuel pump assembly, located inside the fuel tank. To access it, the fuel tank will need to be removed. The sensor sells for around $150 and the labor time guide is around 2 hours to complete the job.
1) Yes. Try a better quality Fuel Cap. Optimally, a GM original, however, I do believe NAPA parts come very close to OEM specs. Ask before you buy. If you believe the fuel cap is good, the light will shut itself off in 50-75 miles of driving. You CAN reset the light yourself by removing the "NEG" cabel from the battery for 15min. HOWEVER! At this point, i wouldn't suggest that until AFTER you have the codes re-read to see what's stored in the memory right now.
2) If it does not work, yes, have it smoked. It's the quickest way I know how to find a leak. Why kill yourself looking for something you may not be able to see. Let the smoke do the work.
3) See (1) above. Yes. Re-read the codes, know "for sure" what you "need" to do.
With ANY EVAP code, no, you are doing no damage what-so-ever. An EVAP code is strictly for emissions and has no bearing on engine performance at all.