The problem with re-using the fluid is that if any dirt at all gets into it you can wind up destroying the transmission. A new transmission costs a lot more than new fluid. If you did reuse the old stuff you would have to take pains to keep as much dirt out of it as possible and filter it before putting it back in. Still, you're talking about something like $126 for 7 quarts of fluid versus the possibility of thousands in repairs if some dirt or grit gets past you and into the transmission. I don't think it's worth the risk.

Never re-use a gasket unless it is designed for it, like Lubelocker gaskets.

With only a 100 miles I wouldn't see any reason not to re use it. Personally I would never reuse a gasket. 

I just finished repairing my transmission and put it back together with brand new fluid and transmission pan gasket. Now, 100 miles later one of the solenoids I just replaced that was brand new has failed. I am wondering if I can re-use the fluid I just put in and the gasket to repair it again? The gasket is a paper-like material and not metal or rubber. I do not want to buy new ones as the fluid is about $18 a quart and the gasket is another $30. When I need 7ish quarts of fluid this adds up quickly. 

Also I drive a 2006 Nissan Pathfinder.