How can there be AES-256-GCM, when GCM is defined for 128-sized blocks?
AES has a block-size of 128 bits in all its variants. The number in AES-128/192/256 is the key-size. Rijndael, the block-cipher that became AES, also supports 256 bit blocks, but that part was not standardized as AES.

Since the block-size is 128 bits, GCM works exactly the same way for AES-256 as it does for AES-128.