I sometimes get a message on my DIC saying to "tighten gas cap" I bought a new one not from dealer, from autozone. Sometimes when I go in the morning to drive I get the message it use to happen once a year now it happens almost every week now.
Check the plastic connecting strap that keep the cap from falling to the ground. I have had a slightly loose cap several times due to the strap pulling counterclockwise on the cap. I always hold the strap back when I tighten the cap so that the strap is pushing the cap tighter instead of the opposite. Also carefully inspect the top of the filler neck.
Most times the problem is not the gas cap. This happened to me and after changing gas caps, I finally had the dealer run a code and it was the evaporative part in the emissions that was the problem. I think 75% of the time, this is what this means when "tighten gas cap" shows up in the DIC. Just wanted to offer this up.
Trace out the EVAP system - from the throttlebody purge solenoid back to the charcoal canister, solenoids, and tank vent fitting. ANY air/vacuum leak in that purge system shows as a "loose cap", among other faults/codes. Also, on some of these EVAP systems there's a test port in the engine compartment, covered by a green (tire valve) cap, on the vacuum line from the throttlebody.
Same thing happened on my 07 Escalade around 100K
I replaced both the evaporative canister purge solenoid and the vapor canister purge valve. It was probably only one that was the problem but for the cost and time I did both. Parts cost under $100, took me 2 hours total.
Canister purge valve is on evaporative tank above spare tire, purge valve was on the intake manifold.