In AES-GCM, from what i understand, the plaintext(as well as AAD and IV) bit length taken in must be a multiple of 8 (as specified by NIST Spes. Pub 800-38D). The AES-CTR however, doesn't "care" about the bit length.
The majority of my confusion comes from looking at how the AES-GCM works as a whole. It seems to me that to appropriately generate an authentication tag it needs two strings of plaintext. Additionally , the GHASH function seems to take in n 128-bit ciphertext strings.
Which leaves me with the question: What is the minimum amount of plaintext data required(after any padding) to appropriately encrypt and authenticate using AES-GCM?