Gas Smell when Heater on @idle

[gale]

We have a customer that has a 2015 Chevy Tahoe and about 2 yrs ago, he started getting a strong Smell of Gas coming Through the vents when sitting idling, In the Winter when the heater is on, but when start moving the smell goes away, but is only when it is sitting idling or sitting a red light in drive and foot on brake -- we replaced the gaskets and pipe underneath the Intake (according to the TSB) did not make a difference. check everything for any signs of leaking no leaks detected.
Just wondering if any one else has had an issue like this and what they found

Gale