Possible 1891 entry for Maggie:
Robert Gibson 59, farmer b. Lanark
Robina Grant 19, niece b. Moffat
Maggie Grant 17, niece b. Moffat
John Grant 8, nephew b. Moffat
David McFarlane 33, servant Ploughman
Address: Howslack, Moffat
Maggie seems to be fairly consistent with her age and place of birth. Possible birth entry for her on IGI:
Maggie Gibson Grant b. 19 October 1873 in Moffat, parents William Grant and Margaret Gibson.
1881 entry and likely Maggie with her family - everyone showing as born in Moffat except for father:
William Grant 50, schoolmaster b. Dumfries
Margaret Grant 46
Janet Grant 19
Jane Grant 11
Roberta Grant 9
Maggie Grant 7
William Grant 4
Thomas Grant 5 Months
Address: Annan Water School House
But, a stage at a time

Keith, you need to work back step by step to be certain. John Clark and Maggie's marriage cert is a good place to start to work back to their individual families.
Your Peter's middle names of Gibson and Grant seems to connect well with Marion's side of the family rather than potential surnames of his father I would think....but, most often, you never know for certain with these types of births unfortunately.
Monica