From Barmouth, head south over the Barmouth Toll Bridge -
its open to motorbikes and pedestrians but not cars.
When i went over it was £1.40 return (July 07).
Some nice views from the bridge and something different. Surface of path to and from bridge is good (you do not need a trail bike) and once on bridge be sure to ride slowly as its a sleeper bridge and not very wide for you and pedestrians.
Once on the opposite side of the river, turn right onto the A493 toward Tywyn (away from Dolgellau). Some nice views from here over toward Barmouth and as far up to Portmerion.
Follow the road to the next major junction, with the B4405