Trouble with a gouge... let me see if I can describe what i'm imagining, and this always gets me in a tight spot.
The gouge is a hollow shafted blade, beveled on the OUTSIDE. With a gouge, you will be able to cut only so deep before this bevel binds on the sides of the grooves.
So you will need a gouge that is SMALLER than the desired groove width. This means you will need to make several passes to get the right width and depth. 'Several passes' means that I'd wander a bit while cutting. Then you need a long straight object with teeth or grit to straighten things out.
It's all tricky business, but keep your head on while working, and stop cutting when the doubts creep in. You'll be OK if you don't rush things and think all the time about what you're trying to do.