Use stepper bits instead of a drill bit to cut the holes bigger in the gasket:
Using a drill bit will bite in and twist the metal causing it to pull up and "rip" things as it does.
The stepper bit will chip away at the material without causing it to pull up, and will simply widen the holes.
Put the gasket against a block of wood to elevate it and allow the weight of the bit and drill pressing to push the gasket flat against the wood while cutting. Will keep things flat as the bit chips away at the holes.
It would be ideal to use a drill press for this to slowly chip away at the gasket holes, but it can be done with a hand drill using slow pressure.

Using a drill bit will bite in and twist the metal causing it to pull up and "rip" things as it does.
The stepper bit will chip away at the material without causing it to pull up, and will simply widen the holes.
Put the gasket against a block of wood to elevate it and allow the weight of the bit and drill pressing to push the gasket flat against the wood while cutting. Will keep things flat as the bit chips away at the holes.
It would be ideal to use a drill press for this to slowly chip away at the gasket holes, but it can be done with a hand drill using slow pressure.