Here's what my father found out 30 years ago:
Father said that there is a tradition in the family that the name was not originally Scottish, but there was no agreement as to its origin. One of his father's books, "The Scottish Nation" (or "The Scottish People") spelled the name with and without the "u" and stated that, in Gaelic, its meaning is "stranger" or "foreigner". This is mentioned in an article (Toronto Telegram, 1970) by Rev. James McGivern which states "Gaul, Gauld, Gault, Gaut, etc. is an Aberdeen Perthshire name from the Gaelic "Gall"- a stranger. Aberdeen adjoins Banff. The village of Cullen in Banffshire seems to have been the centre from which the Galts spread east and west. Being north sea counties, Moray, Banff and Aberdeen would have been frequent victims of Viking raids. The admixture of Norse peoples with those of England, Ireland and Scotland began about the ninth century. McGivern calls attention to the Viking name Gaut or Gaute which he says is the name of a family of Gautungs in Norway, Galte in Norwegian means bear,Galtung means young boar. So the Scottish Galts-Gaults probably were foreigners from Scandinavia, as were many other Scottish families, for example the Gunns, Kerrs, McLeods, Lamonts and others.
McGivern records that in 1343 a man named Sigurd Gautesen called himself Sigurd Gautesen Galte and used a wild boar decoration on his sword handle. His nephew, Gaute Erickesen used the wild boar as his seal, which is in the Norwegian National Museum.
from Jean Galr Toronto On. Canada firstname.lastname@example.org