AES-GCM, like all versions of AES, has a block-size of 128 bits, or 16 bytes. Generally speaking, you shouldn't have to worry about the "merging encrypted blocks" as the implementation should handle encryption of data larger than a single block for you, and simply provide you with the full ciphertext that is the end result.
Likewise, you don't have to worry about generating the authentication tag. The implementation generates the authentication tag for you. and you simply need to retrieve it once encryption is complete.