Question says it all. Will there be a problem using a stainless steel bolt with a hot-dip galvanized nut and washer? This will not be a load-bearing application, will be exposed to outdoor conditions (snow, rain, etc.).
Depending upon the thread cut, it may be a sloppy fit even if the threads per inch, and diameter are the same. I believe it is galvanized after the threads are cut, therefore the threads might be cut deeper before the galvanization. Dissimilar metals don't often get along, but if it is a non-critical application and the nut is not a sloppy fit, you should be okay.