Minimum plaintext length for AES-GCM
The original proposal specifies:


  A plaintext $P$, which can have any number of bits between 0 and $2^{39} − 256$.


The NIST SP 800-38D specifies:


  Although GCM is defined on bit strings, the bit lengths of the plaintext, the AAD, and the IV shall all be multiples of 8, so that these values are byte strings.


So there is a clear difference between the original proposal and 38D. I presume that this is because the test vectors almost never contain any tests for plaintext (or AAD, IV and authentication tag) that are not a multiple of 8 bits. In that case it is probably better to leave it out.

So onto your question:


  What is the minimum amount of plaintext data required (after any padding) to appropriately encrypt and authenticate using AES-GCM?


Well, zero bits, obviously. GCM doesn't require any padding of the plaintext before it is used within the algorithm. Steps 4 &amp; 5 in 7.1 of 38D define how the input is padded before it is used. Although the specification is limited to accepting bytes, there is nothing stopping you from creating an implementation that operates on bits.