No spark plug comes with a gasket that I know of. If you mean the crush rings, they're supposed to be there.
I've said in the past that an aftermarket ignition system can increase performance, but I don't think that's a good place to spend your money either. If everything is within spec (and old stock wires usually will not be, nor the spark plugs, or the distributor, etc.), than there's no need to try to "improve" anything really.
If you increase the spark gap, the spark will be longer, but won't be as "thick" (there's a technical term for you!
), or not as hot. The optimal gaps are listed in several manuals including the haynes workshop manual. I wouldn't mess with these.
Ahmet