Does it look like the one pictured below?
http://www.sega-16.com/forum/showthr...or-the-Sega-CD
I don't know about value but I believe that gun is even rarer than the pink Konami gun. There weren't really too many light guns made for the Genesis, I'm not sure what else it could be.
I went out for yard sales earlier but didn't find anything. At one sale there was a Game Gear with games but nothing looked interesting so I didn't bother asking a price. At another place that advertised a sale nothing was set up outside at all, this was at around 10:30am so I'm not sure what their deal was. Maybe everything was inside but I didn't feel like stopping to ask. Not much else as it looked like it would rain, though I don't think it did. I just went back home and stayed in all afternoon.