Implementations of GCM with 64-bit cipher
This would not be recommended in most cases. Note that AES-GCM fails terribly if an IV repeats. In a 64-bit block cipher, if you want the probability of such a failure to be below $2^{-32}$, then you would be very limited in the number of messages you can encrypt. If you can guarantee a unique IV each time, and are not using a random IV in order to get uniqueness, then maybe it can be OK.