Thanks for the reply. The 'gasket' in question was indeed 'designed' that way and is sold only as a 'unit'. It's a MP valve used on Pentair filters, as well. It's similar to plumbing wax, like on a bowl ring. I found one online that I can get for $40 with shipping, but I'll kick and scream every time when paying too much for poor engineering. I may hold off and get a wax wing and just improvise it and re-make what was there - I dunno.
More bothersome is the possibility it may be the handle spring, as it is quite rusty, and the up-down action is sluggish - the valve may have been getting stuck with the handle in a semi-depressed position. I don't know enough about the inner workings of a MP valve to hazard a guess. I'd hate to end up paying more for internals trying to fix it, than just buying a new top half of the valve ($80).
As an aside, and maybe another symptom of said handle-sticking, is that pressure is high at the valve (water was leaking between the upper and lower halves of the valve - O-ring, or something else?). Checking by hand, return pressure seemed to be really high at the return jets, too. Doesn't make alot of sense, as with good return you'd think pressure would be low at the valve. The winter-damaged pressure gauge is minimal help - it starts at 10 with the pump off - but water shouldn't be leaking out of that joint, in any case.
Anyways, thanks again for the reply. Maybe someone else will read the above and say "A-ha! I know what it is!"