The answer was linked to above. Here you go again one last time
GOP frames set to 25 so I am not understanding how that would be I-frame only?
It is not. Lossless does not always mean I-frame only. Lossless H.264 with GOP=0 or 1 is going to be heavier than what the Convert dialog does, which is x264 crf15 = 70% Quality.
When I google it there is usually two values, m and n
That is a legacy convention/notation from MPEG-2. Basically, N is GOP, and M is the number of B frames + 1. When GOP is 0 or 1, the B frames option is not applicable.