I recently put a GT40P in my 66. I did a bit of reading on distributor gears. It's a bit confusing. If it's a Ford factory distributor from a roller cam motor, no. The factory cam is 5150 billet steel. You must use either a steel gear or Melonized gear with it. Ford uses a steel gear and not a Melonized gear. You can however use a Melonized gear on a cast iron flat tappet cam. I read an article where it was stating that most, maybe all aftermarket distributor companies use Melonized gears as the default material as it will work on anything with the exception of mechanical roller cams since they are 8620. Melonize is a hardening process. Aftermarket hydraulic roller cams are for the most part "SADI" core cams which are ductile iron and just the lobes are hardened. You can use a flat tappet distributor gear besides the steel or Melonized gears.
The gear must be a slight press fit. It can not be loose and as gypsyr said there's 2 different sizes. The flat tappet distributor is something like .467" and the other is something like .563" I think. Do a search on my started thread stats and look for my GT40P post. I cover how to swap gears.
I'm not a complete idiot, pieces are missing.