Problem: Build-up of gas inside dryer drum during operation.
Vents are clean and clear. No leaks from exterior gas piping connected to dryer. Ignition glow plug operates normally during cycle.

Unfortunately it's a bit hard to diagnose like that. Is it a natural gas smell or a burnt gas smell. The only thing inside the dryer is the gas valve. So if you are sure that ALL connection are tight, including the IP threads and the flare fittings, it must be the gas valve if it is a gas smell. If it is a burnt type smell, it could be a disconnected vent inside or possibly a clogged vent line inside the dryer. Give us some more detail.