Carlisle and Newcastle have been called this!
But in Scotland, almost every town with a road running to the north claims to be the 'Gateway to the North', or 'Gateway to the Highlands' !
It depends rather on the context - in history, any township likely to hold up the progress of an invading English army would be given the title.
More recently, the term became popular through tourism. But this didn't start until Victorian times and the coming of the railways. Perth became a very large railway centre, and was widely advertised as being the 'Gateway to the Highlands' - with some justification. It retains this title today, though the local council has seen fit to use 'The Perfect Centre' as the modern town motto.
I feel that 'Inverness' is pushing this a bit far - it's already in the north!