Start by forgetting about Ancestry. It won't tell you everything you want to know. It might come in handy for looking up census transcriptions but it doesn't have all the birth, marriage and death information.
You need to use Scotland's People
www.scotlandspeople.gov.uk. See
https://www.rootschat.com/forum/index.php?topic=714261.0A Scottish marriage certificate from 1855 onwards tells you the full names of the parents of the couple, including their mother's maiden surname.
John Gilchrist and Margaret Turner were married in Old Kilpatrick in 1918
Robert Turner and Mary Henny were married in Gorbals in 1875
Peter Gilchrist and Sarah Holmes were married in Dennistoun in 1887
The next generation back must have married before the start of civil registration in 1855.
So the next port of call is the census. Robert and Mary should be in the 1881 census, and you can view the transcription made by the Latter-Day Saints free of charge at Scotland's People.
Peter Gilchrist, 26, born England is in the 1891 census at 46 James Street, Glasgow, with wife Sarah, 25, born Glasgow, and sons James and Peter, also born in Glasgow.
From FreeBMD
https://www.freebmd.org.uk/cgi/search.pl James Gilchrist and Elizabeth Finney were married in Liverpool in 1862. Their son Peter was born in 1863 in Birkenhead. They must then have moved to Scotland because they had James in 1865 then Rose in 1869 and another Rose in 1870, all in Glasgow. All three of these children died in infancy.
In 1871 James Gilchrist, 29, born Glasgow, is in Glasgow with wife Elizabeth, 29, born in Ireland, and Peter, 8, born England. There are also two daughters, Mary and Lizzie, both aged 11, born Glasgow. So it may be that James had been married before. Or they could be illegitimate children of either Peter or Elizabeth.
