I have a GE gas dryer, model DWSR405GBOWW. It is plugged in but not hooked up to a gas line. When I turn the start knob to the start position the dryer turns on but the knob springs back to the off position and the dryer turns off. Do I need a new knob? Do I have to have the gas hooked up to keep the dryer on? Help!
Your knob is fine. The starting contacts that are shut by that knob are typically in parallel with (or "short around") other contacts inside the drum motor. Hold it shut momentarily to start and allow the drum to come up to speed at which time sustaining contacts in the motor should be shut, allowing you to release the knob.
The only drier schematics that I have right to hand have the start switch across only the drum motor. Once that motor is up to speed a set of centrifugal contacts shut to keep it running after the knob is released. The motor has a thermal fuse and overload protector but if those are open then the motor shouldn't start at all. The heater-associated parts (gas valve, igniter, thermostat, etc.) are energized by a second set of contacts in the drum motor centrifugal switch but don't affect whether or not the drum motor runs.
So, my guess without seeing the actual schematic for your dryer is that you have a bad centrifugal switch in the drum motor. Fix is probably to replace the motor, as the switch is "inside" the motor housing and not really separately repairable.
A few videos that give some feel for where the centrifugal switch is and what's involved in testing and replacing the motor. Quite do-able if your Possum Lodge membership is up to date.
His dryer should have a "Fluff" no heat cycle, and "wrinkle care" settings on the dryer timer, that would not need heat to operate. He should try operating it on those settings where the gas would not come on.