I have never believed the phrase means 'it's a thought that would make our rats,(or people who behave like rats)'?? do what?
The sense seems to be that whatever evil happening is blowing through the great glen..., it should upset or frighten us.
Usually the Scottish word 'gar' means to 'make' or 'force' but it has another meaning of 'filth' or 'slime' and I think the explanation 'turn our stomachs' makes more sense.
Jim Mclean