Hello Capt. Jock,
I did a quick search of the IGI for both Swan/Swann and Gault/Galt in Scotland.
Below are the areas with more than 50 of each name -
Some areas had many more than 200 named.
Gault/Galt - Aberdeen, Angus, Ayr, Banff, Fife, Lanark, Midlothian, Moray, Perth, Renfrew, Stirling.
Swan/Swann - Aberdeen, Angus, Argyll, Ayr, Berwick, Clackmannan, Dumfries,
Dunbarton, East Lothian, Fife, Inverness, Kirkcudbright, Lanark, Midlothian, Orkney, Peebles, Perth, Renfrew, Roxburgh, Stirling, West Lothian, Wigtown.
As you can see Swan/Swann was more widespread than Gault/Gault.
Someone with more knowledge of Scotland than I may be able to make an assumption of the name spread.
Regards
Leigh