If Andrew is his father (and I think he is, but as yet no proof) his mother would be Janet Wilson.
Unfortunatly siblings are where this branch breaks down a bit for me as I have two John's, John Gammie b. 27 Jul 1740 & John Gamy b.29 Jul 1946. This is not impossible but I have seen both John's as heir to your line and can't prove either as yet. (I have briefly play e-mail tennis with both Alan Morrison of Glasgow and Stewart Gammie of Canada as well)
Other siblings for Alexander include William b.10 Jul 1744, Helen (Gamy) b. 21 Apr 1746, Joseph (Gamy), James (Gamy) b. Jan 1751, Peter b.Apr 1759, Elspet Feb 1762.
I must stress though that I have not verified any of these other than they are in the Church of LSD's index which everyone tells me is incomplete anyway.
If you would like to compare notes feel free to drop me e-m to email@example.com
and I can send you my line as I see it.