This map from Oregon Parks shows the best bike route from Troutdale on the west end of the gorge to The Dalles. As to your original post, it shows one section of riding on the interstate shoulder, but mostly you'll ride on the old scenic road.

Another thought I had since you mentioned riding the gorge both ways was wind. The winds in the gorge are strong. Riding both directions means that you'll likely have a tailwind one way and fierce headwind the other. Winds can be from the west, or from the east and really strong. Be prepared for this.

This map is in sections, so scroll down to see each section http://www.oregonstateparks.org/ckFi...H_BikeMaps.pdf