P0440 Chevy1500Expr VAN 6c4.3l/what-where

For a 2001 Chevy Expr1500/6cyl/4.3l HELP! Emissions mechanic is out sick, can't find a rpr. manl to buy! in stores and insp. is due? Running a P0440 and Po440 P for DTC's.
Have gas (not quite clean, not ahrdly full exhaust smell) sometimes at startup, or backing up,(can smell it outside the veh. w/ window open) -and when running engine in park with heat/blower on for any length of time get an overpowering slightly MORE burnt fume IN vehicle. that crreeps up(-head is swimming right now)! (ALSO, travel with a Disab. Support animal who is sleeping after vomiting) Erased code returns after 5-10 ignition cycles/check engine light comes on and stays on. (Recently corrected Camshaft position sensor Code by replacing it. No more DTCode showing for that one). In 6th- or less could I get a clear parts/progression of what EVAP emissions control system is/does and/or better yet what these parts look like and where I can find them if I want to check the whole chain of what can be setting this code off ? (I HAVE read the other posts that might pertain to this already, but not for the above mentioned veh. make model year). ALSO, just replaced fairly charred sparkplugs with no change in DTC or fumes.
-hb
PS-WE HhhhAaaaave the fuel filter......(as any reference)!?