At the NAPA website, I searched for '64 GTO Oil Drain Plug Gasket, they came back with 2 choices.
Both are Nylon.
Fel-Pro 70822, same as the one Brad linked from AutoZone.
The other was from NAPA Solutions p/n NOE 7041078.
My local store had both in stock. The Fel-Pro was in packaging that probably added a dollar to the price.
Online it was priced at $1.33 but in the store it was more than $2.
The NOE was $0.79 online and in the store,
I chose it because it has a larger OD, 0.988" vs the Fel-Pro at 0.900".
See the pix, the NOE seems to be a perfect match for the OD of the plug flange and slid on easily over the threads.
Perfect size I think.