Honing guide for gouges
Plane blades and chisels are far easier to sharpen than gouges, just because a honing surface that fits the shape of the cutting edge (flat!) is easier to come by. In-cannel gouges can be honed with a variety of cylindrical devices, such as leather-wrapped dowels, that can be made to match the edge with little difficulty. Honing the outside of a gouge means solving the problem of finding the shape to hold the compound that is precisely the shape of the cutting edge of the gouge.
That shape can best be defined by using the gouge itself to form it. Choose a piece of straight-grained, knot-free wood, the softer the better. Cut a channel into it, using the gouge that is to be honed. Cut the channel deep enough so that the entire cutting edge is below the surface of the wood. The cross section of the channel should be the precise shape of the gouge seen edge-on. Then rub a generous coat of Veritas blade honing compound onto the surface of the channel.
Pressing the gouge into the channel so that the outer surface at and behind the edge bears firmly on the wood, drag it backward along the channel a number of times, until the edge takes on a mirror sheen. If the inner surface has been given a proper honing, you will have a razor-edged gouge, ready to use.