If fresh water penetrated the gauge (dial area), then it might be possible to dry it out, fix the leak (replace plug, replace face, install a new o-ring, replace the case, etc.), and the gauge will work OK.
If salt water got in there, you can attempt the resuscitation outlined above but chances are the corrosion will have already begun...and it's just a matter of time before the corrosion makes it unusable. If salt water has been sitting inside the gauge for some time, then it's probably a goner.
For most people, the solution is just to buy another gauge. I would certainly take it to the shop where you bought it from. Perhaps they'll provide a replacement free of charge as a warranty replacement. If you bought the gauge secondhand, then just get another one. If you're really pinching pennies, used gauges can be acquired fairly inexpensively on Craigslist and Ebay.
New brass-and-glass gauges will cost $60-$80 for the naked gauge. Expect to pay $20-$30 more for a gauge with attached HP hose. FWIW, you can almost certainly re-use the HP hose that's connected to your flooded SPG.