"thus I might wonder how you'd get on speaking Erse in Greggs in Berwick"

But again that isn't the point. When you are crossing the border at Lamberton or Carter Bar you aren't just moving into the Scottish Borders Region. You are moving into Scotland the country itself - hence the Gaelic signs at the border. It is a recognition of part of the nation's culture at the land entrance of the country. Elsewhere in the Borders Region there are no Gaelic signs etc. Why a couple of signs would be an issue for anyone still baffles me.