There are 3 basic type of gas caps:
"NOT VENTED" - As the name implies. Seals tight. Used on cars with another method of venting the tank.
"VENTED" - basically, open to the atmosphere - a pinhole, etc. I suppose this could somehow get clogged or plugged, but this is hard to fathom.
"PRESSURE-VACUUM" - This type has a small check valve built in, allows unrestricted flow of make-up air in; vents to the atmosphere at a predetermined pressure point.
I have found that this last type covers a multitude of sins (venting problems, raw fuel spillage, etc.)
Rick E.