I'm not sure if I can do this, so I will ask first. I have parts split from a complex model that have planes I don't like. I want planes in the centers so I have a chance to mate these parts, even though there are no planar surfaces. Can I open the gasket in sketch, and move it so the origin is in the center? If I do that, does it affect anything else?
Andrew,
If this gasket is to be mated in the same location where it was created relative to the other bodies, you should be able to mate like planes to position it. Or better yet, for the future, if you select the Insert Component feature, select the part, then click the green check mark without clicking in the graphics area, the component will be fixed with it's three primary planes aligned with those of the assembly.
If that won't work, I believe I'd open the part and create new planes to use for mating instead of going back and trying to edit the sketch that created the feature. You may need to place some sketch geometry to use when creating the planes.