I never put the gasket on the nut side, because it's not made to take the friction of the nut spinning on it. That said, if the gasket is in good shape, and the surface it's on is clean and smooth, it will seal well...regardless of whether it's on the inside or outside of the tank. In other words, you can put the bulkhead in from the inside of the tank, or outside, nut inside or outside...but just put the gasket on the flare side, not the nut side.
I have used some "food grade" teflon pipe dope on the gaskets before, when I had sealing problems due to an imperfect gasket or surface.
Thank you! I will try that. I am hoping the gasket is usable.