You most likely have 2 separate problems here...
When you took the car to Pep Boys they most likely connected a scan tool or a code reader tool to the car's data connector, and received a fault code related to an evaporative emissions system leak. There are many fault codes that can store for evap system problems, but none of them specifically mean that a fuel cap is leaking, so I suspect they basically guessed at the cause, sold you a fuel cap, and cleared your computer memory and sent you out the door.
Your trailblazer has a complicated system of tubing, valves, and solenoids that are there to capture any gasoline vapors from the fuel tank, store them in a charcoal canister, and then when conditions are right allow them to feed into the engine to be burned. The plumbing for this evaporative emission system extends all teh way from the fuel tank up to the engine, and also includes teh fuel tank itself, the fill pipe that leads to the gas cap, and the cap. When youa re driving, your car's computer runs a self test of this system to check operation of all of the electrical components and to check for leaks; it does this by using engine vacuum to draw a vacuum on the fuel tank, then seals it off by closing some solenoid valves, and monitors the vacuum in the tank using a pressure sensor mounted on the tank. If the vacuum in the tank degrades too quickly, the computer software identifies this as being caused by a leak somewhere in the system and stores a fault code; different codes are also stored depending on the size of the leak. This self test is extremely sensitive; it is capable of detecting a leak the equivalent of a .001 inch pinhole anywhere in the system.
Testing the evap system to locate the source of the problem does require some special test equipment, including a smoke generator machine used to fill the plumbing with a high density smoke to locate leaks, and also a GM factory scan tool or equivalent to run self tests on the system and to force teh leak test to run to see if it is fixed after repairs are made.
Something to keep in mind is that the mechanics employed at discount chain stores like Pep Boys generally are not top notch technicians; these types of employers generally hire less experienced mechanics with little formal training and pay them a low wage. As a result,m they usually do not have the training or the test equipment needed to accurately diagnose this type of emission system problem. Generally, they usually guess that the fuel cap is bad, sell you one, and get you out the door. Once teh light has been turned off, it will usually stay off foe a few days untill the car runs it's self test again, at which point it fails and the light comes back on.
This is not really the type of fault you can accurately diagnose at home without the special equipment required; for that reason you will likely need to take this one in to a GM dealer, or a shop that has the proper test equipment and technicians specifically trained in GM evap system diagnostic procedures to have the actual cause of the fault found. Usually it turns out to be a very small pinhole leak in a piece of tubing somewhere, or one of the various valves not sealing completely when commanded to close, so repair usually is not very expensive.
In regards XXXXX XXXXX other symptom you described, the hesitation and jerking on deceleration, this is something unrelated to the evaporative emission system fault. Evap system leaks will not cause this type of symptom; usually with an evap leak the driver never notices anything other than the light being on, since the evap system is not related to how the engine runs.
To find teh cause of this second problem some degree of diagnostic testing will be required as well, beginning with using a scan tool to see if there is any fault information stored in computer memory related to that problem. There are many possibilities as to what could be causing that particular symptom, ranging from an ignition misfire to a vacuum leak to a torque converter clutch problem. So, my recommendation would be to take it in to a qualified shop to have the evaporative emission system problem diagnosed, and while it is in there ask them to see if can identify the cause of the second symptom as well while they are checking the computer for the evap problem. Without any concrete information such as fault codes to begin with, it is not really possible at this point to accurately identify the exact cause of the jerking on deceleration problem.