P0449 is not a leak code, so replacing the filler cap would be a waste of your money, in my opinion
This code often occurs because the evap vent valve is plugged with dirt, spider webs, etc.
I seem to remember your Yukon is 2007 model year, correct?
If so, this bulletin applies to your vehicle
Mind you, I’m not there with you and your truck, and what’s described in the bulletin might not actually be the cause of your problem.
But we have lots of Tahoes and Yukons of your vintage in our fleet, and this modification WAS necessary on many of them. These guys do drive off road and get there evap system all full of crud. I’m not sure what kind of driving situation you have, though.