Janine,
When I tried a google for "When was Gavin Young minister of Ruthwell, Dumfries-shire?" quite a lot comes up on a list. 5th site from the top gives good photos of Ruthwell church and the Ruthwell Cross. It says that Gavin Young hid the cross under the floor of the church to prevent its destruction.
On another site, about the Carlyle family, on this list, it says: "Edward Carlile of Limekilns, the eldest son, married Margaret, daughter of Gavin Young, minister of Ruthwell before 1699. I wondered if the Carlyle family tree would reveal anything more of the Gavin Young's children, but maybe Margaret is the only one they are interested in. (However, I don't know if the Carlyle descendants have a tree online at all.)
It seems that Gavin Young's wife was Jane Stewart (you will already know that) - but it gives this a mention if you click onto page TWO of the original google list. It is the site 6th from the top.
If it was me researching it, I would make up a variety of googles, and try them just to see what comes up. For instance:
"Gavin Young, minister of Ruthwell, Dumfries-shire - how many children did he have?" Or: "Names of the children of Gavin Young, minister of Ruthwell." Or: "Ancestry Family trees who have Gavin Young, minister of Ruthwell, born 1677." Etc, etc, etc - - I am always amazed at the information that comes up if you type in googles, no-matter how vague they are.