It's not unusual for the names Gavin and Guy to be used interchangeably.
Both marriage certificates are available to view online at
www.scotlandspeople.gov.uk at modest cost. They will tell you the full names of all the couples' parents including the maiden surnames of their mothers.
That death in 1975 is Ann Waterson Hamilton, other surname McIntyre, mother's maiden surname Cumming*.
The index to the 1911 census lists two Annie McIntyres of the right age.
One in Kilbride aged 10, with Margaret, 37; John, 8; Janet, 6; and Willie, 4. This Janet's age is exactly right for Janet Taylor McIntyre or Jack. Janet Taylor Jack was born in Kilbride in 1904. So it looks as if this is the correct family.
Once you have Ann's parents' names from her marriage certificate, you could look at the original 1911 census records and see what they tell you. You could also look for and view the marriage certificates of all four sets of parents, assuming that they were married of course.
Ann isn't indexed in the 1901 census, and she is listed as 9 in 1911, so she must have been born after the day of the 1901 census. However in 1901 there is a family on Holy Isle, parish of Kilbride, consisting of John McIntyre, 72, born Argyll; son John McIntyre, married, 33; daughters Annie, 24 and Marion, 22; daughter-in-law Margaret, 27; grandson Robert Paterson, 5; and a boarder. The next household is that of William Hamilton.
There are two deaths of Gavin Hamiltons in Bute after 1900 - one in Brodick in 1970 aged 80 (so born 1889/1890) and the one already mentioned in Lamlash, who would have been born 1890/1891.
In the 1901 census there here is a Gavin Hamilton, aged 11 (so DoB 1889/1890), at Corrieghills Dhunan, parish of Kilbride, parents Lauchlan Hamilton and Elisa.
Also a 10-year-old Gavin (DoB 1890/1891) who from the 1891 census seems to be the son of David Hamilton and Catherine, also in the parish of Kilbride.
*(Mother's maiden surname was not included in the indexes before then. They are slowly adding this information to the indexes but they started in 1855 and are working forward and are nowhere near the 1970s so far. So this approach won't work for the other people mentioned.)
