Is it an absolute increase of gold or a %. If percentage, I would send every caravan to a gold city.
% increase but each settlement can only send 1 caravan to each other settlement. What I do is always send 1 caravan to my capital and the other 2 to help build road networks. My capital is my gold farm (always take the gold levelup bonus and build the gildar improvements). I had one game a few patches ago where my capital was cranking out about 1,100 per season gildar around turn 250 or so. Each settlement had a route with the capital and I used every available food resource to keep building housing and increasing the population. It was overkill but pretty neat.