I need to pull the drivers side header off to weld a hole in the collector, can i reuse the gasket for the flange, do i need to use a new one?

I'd say you might be able to if it doesn't tear, but you should ask yourself "do I want to pull the header off AGAIN if the old gasket doesn't seal"?

The gasket's not going to tear since it's metal...
I'd reuse it, they're pretty good about sealing back up on stock manifolds.
But, if it's a header and it's a little harder to work on, do you really want to take the chance?