Alright I found the mystery pulsating part "with an electrical connection and a rubber hose" (and yes it is pulsating) but only after finding the rubber hose completely busted off where it attaches to the throttle body...
Once I had the engine cover off and engine running, I could hear the hissing air and tracked it down. I'm guessing that'* the actual problem causing the gas cap warning.
Then after I followed the air line back to the pulsating thingamabob and attempted to remove it (the air line), the rubber elbow snapped off completely. So I think I need to replace the entire line + both ends of connections that run from pulsating thingamabob to the throttle body. What'* that called, and does it come in one piece?
Current GMs: '95 Chevy Suburban C2500, '69 Pontiac Executive
Former GMs: '83 Buick Park Ave, '73 Buick Electra, '92 Chevy Caprice 9C1, '98 Pontiac Bonneville SLE, '99 Olds Aurora, '00 Buick LeSabre