I would say yes there is plenty of great options for a 400 mile round trip tour starting in Portland and going through the Gorge. But you said you don't especially like hills, so there's not a whole heck of a lot to recommend in "flatter" tours that go through the Gorge (or really anywhere else in Oregon). You could keep on riding east along the Columbia until you reach a good point to get transportation back, like the Tri-Cities. But once you get beyond the National Scenic Area which ends just east of The Dalles it's...not as scenic. Also, there ain't a heck of a lot of services between The Dalles and the Tri-Cities, which would make things difficult for credit card touring.

If you do want to put up hills, there's plenty of decent riding around Central Oregon.