It seems I'm missing some piece of trim here  But I wouldn't know what to call it or where to get it.
Halp!

Its the center of the tail fairing.

Shouldn't be too hard to find, quick ebay search found this:

http://cgi.ebay.com/ebaymotors/04-09-Suzuki-GS500-GS500F-Tail-Center-Fairing-Trim-_W0QQcmdZViewItemQQhashZitem20b1ae2534QQitemZ140419933492QQptZMotorcyclesQ5fPartsQ5fAccessories

Mine's an 89, I'll keep an eye out thanks!

The centerpiece for 1989-2000 is hard to find and usually goes for $20-$30 each - about the same price as a side plastic! I guess because they get broken up so easily.
Ebay search for "gs500 (tail,fairing,plastic)" and you should find one.
If no luck, contact me later. I think I have a few laying around though it might have to be painted to match (color should be pearl novelty black from Colorrite.com.) Things kind messed up and stored as garage being worked expanded.