If you read it all in one go like this:

"It's a thocht that will gar oor rottans, a' they rogues that gang gallus, fresh and gay, tak the road, and seek ither loanins ..."

it might make a little more sense.

The whole thing means "it's a thought that will make our rottans (rats, or people who behave like rats), all those rogues who carry on boldly and shamelessly (as Senga said), take the road and seek other places ...

Hope that makes sense!