Most of the articles I've read about sharpening gouges advise spending a half hour or longer rubbing the tool back and forth on an India stone to develop a "wire edge" uniformly all around the cutting surface, and then more of the same on a fine honing stone and then a leather strop to remove the wire edge and to polish the cutting edge to a mirror finish.  I guess that's a good way to do it if you love hand tools and have lots of time.  But frankly, I find it very difficult to hold the tool at the same angle while sharpening it this way, and especially so because of the need to keep rotating the tool to sharpen the U-shaped tip evenly.

Being a pragmatist, with no qualms about using "machinery" to get the job done faster (and possibly better), I recently rigged a setup using a sanding disc in my electric drill which let me sharpen two dozen gouges in a couple of hours - and best of all, it made it a simple matter to maintain the same angle during sharpening and stropping the tools.  Basically, I just laid my drill on its back and attached it to a two foot long piece of 2x6" board that I could slide around on a tabletop.  (I actually placed another 1" thick board underneath this, because I wanted the center of the drill to be about 4" above the table surface.)  I then cut out a circle of #400 grit wet-or-dry waterproof sandpaper to fit onto a rubber-backed sanding disc attachment and chucked that into the drill.  See the accompanying sketch.

By experimentation, I found that placing the disc back about an inch and a half from the table edge resulted in about the right angle when the tip of the gouge was about even with the center of the disc.  (Gouges are typically ground at an angle of about 25 degrees, and it's possible to move the board back and forth slightly, or move the gouge a bit higher or lower on the disc, to match the angle at which a particular gouge was ground when it was manufactured.)  Once the correct angle was established, I started the drill at slow speed (so as to not overheat the tip) and found it a simple matter to grind the U-shape uniformly by simply rolling/twisting the gouge back and forth while resting the round handle against the table edge.  Using light pressure and a little spray bottle to wet the disc from time to time, I found that the desired "wire edge" began to appear in a short time.   Stop grinding and check the edge from time to time.  The wire edge may be visible to the naked eye as a bright edge if you catch the light just right, or it can be seen with a magnifying lens as little feathery pieces of steel extending outward from the edge.  The idea is to continue grinding until a wire edge is developed all across the cutting edge, and ideally the cutting edge will be nice and straight across from one side to the other if you've rocked the gouge back and forth properly during grinding.

Once a uniform wire edge is achieved, I use a ceramic "carver's slip" to break off the wire edge.  This carver's slip is about the size and shape of half a soda cracker, very smooth and hard, rounded on one of the long edges to fit inside a U-shaped gouge and tapered on the other to fit into a V-shaped gouge.   The wire edge is broken off readily by using very light pressure to try and take a slice of the carver's slip with the gouge, first inside the U and then outside.  At this point the gouge is as sharp as it's going to get, but its cutting action can be improved still more by polishing out all those small scratches left during the grinding with the #400 grit abrasive.  To do this step for the two dozen gouges I was sharpening, I went back to the drill setup - replacing the #400 grit sandpaper with a leather disc cut from an old briefcase, and "loading" this leather "strop" with a very fine polishing compound called "tripoli".   Just a little polishing produced a mirror finish that lets the gouge just glide across the material it's cutting.

Once a gouge has been sharpened and polished as just described, it shouldn't require re-sharpening for a long time unless you drop it on its tip, or try digging with it or something like that.  I do find, however, that it helps to "touch up" the gouge on a strop every now and then when carving - say every half hour or so - whenever you begin to notice that it isn't cutting as smoothly as before.   For this purpose I just use a scrap of leather I cut from a thick belt.  I have a bar of Tripoli polishing compound that I rub onto one side of the leather for stropping the outside of the U, and then I rub some of the polish on a rounded edge of the leather for polishing inside the U.  A few strokes on each side usually makes a noticeable difference.

Sharpening supplies (or even sharpening services) are available from Woodcraft, from whom I buy most of my carving supplies because they've given me good service.