Hi Andrew - Welcome to Rootschat!

I can verify there are several McAdam families in the area at that time. There also seems to be a fair amount of movement between Penninghame, Minnigaff and Girthon for most families in the area. It is only a couple of miles to cross parish boundaries so you may find a marriage in one, children's chirstenings in another and deaths in another - this just means checking both Kirkcudbright and Wigtownshire records.
Do you have any idea of roughly what George's birth year might have been from census records?
The only obvious entry on the IGI is 1817 Penninghame, but the wrong parents. They could still be related though, possibly cousins.
If you look for the Death certificate of George it may well contain information about his parents and birth year, provided that the person registering the death knew them. You can access this on Scotland's People for a small fee. (£6 for 30 credits - 5 credits per certificate, 1 for search results)
There is one possible candidate showing up - A George McAdam died in Wigtownshire between 1870 and 1880 aged somewhere in his 60s (I didn't bother narrowing it down further as I don't have any credits to check just now)
If you have any further information you could share it might help us help you - unless you want all the fun of doing it yourself!
Kirsty