Scotlands People Website (Government-run Birth Death & Marriage records) free to search but you have to pay t see images.

It records an Isabella Muir Gunning born 1888 registered in Barr. The appears to be only one Isabella Muir Gunning in the marriage records. 1924 to William Bryce Boyd registered in Kirkcowan. There is an Isabella Muir Boyd registered death in Ayr in 1963. This is likely her death as you can search with maiden name as well and only returns this death in the whole of Scotland. Finding children within the birth records would be difficult as all you have to go on is surname Boyd post-1924. If children didn't migrate you would search deaths of Boyd with mothers name Gunning. This returns one result post-1974 (Mother's name not searchable pre this). William Bryce Boyd aged 63 at death in 1989 registered in Ayr. There could be others who are still alive.