This is a well known, common problem with '60s Gretsch drums. One solution is to buy the American Vintage heads from Aquarian, which are slightly oversized and may work better. Another option, if possible, is to go to your drum store and try
every head they have to find one that fits. I've also heard of drummers breaking the aluminum hoop on the head to allow the head to fit. The last, and worst option, which many have done to Gretsch in the past, is to cut a piece of the pearl off the drum, so the head goes right on the wood shell. (I'd try the other options, especially the Aquarian heads, first!)