What is the U6 promoter transcription start site?

The “G” that you mentioned is in fact the “preferred” transcription start site, however, this can be replaced by any other nucleotide. It’s just that U6 prefers “G” as the first transcribed nucleotide and that makes the transcription more efficient. You will find many examples of this in vectors available for transcription of shRNA or gRNA (for CRISPR) system. Just look for a restriction enzyme after U6 that is used for cloning a short RNA gene. You’ll figure out the exact position of the transcription start site.