Mine fell off somehow 

Same thing happened to me  I believe you can buy the knob and screw you need on bikebandit.com, they are an excellent site for OEM parts!

Yeah, I'm missing mine as well. Gave up on finding one out here in Europe.

You don't have dealers in Europe?  On the OEM microfiche the parts are only like $7 for the knob and $1 for the screw maximum.

US sites include...
Partzilla.com
Rockymountainatvmc.com
Ronayers.com
Bikebandit.com

Part numbers differ based in model year.  Just click OEM parts->Suzuki->motorcycles->model year->GS500(E or F).  That's pretty much universal to any site.

We have some websites but none of them sell that knob...

www.wemoto.com   
Go to "Search for OEM Partno".  Just enter the part numbers you need and you can order from there.  Part numbers are as follows for the different years...

Screw is the same for all years - 34124-30B00

KNOB
2004+ - 34124-01DA0
2001-2002 - 34124-21E10
1989-1999 - 34124-30B00

I'm suprised at the lack of ease of ordering parts across the pond.  It's so easy from here in the US with all the dealer sites having microfiche available online that you can order just about any part from.