We have a support van with a mechanic and tools but it is nice to know you can do a lot of stuff yourself by the side of the road if you need to. It should also help when we are doing the half-way service on all the bikes in Dushanbe.
Tyre levers, inner tube and air pump for punctures. Spares for the service (filters, plugs, bulbs etc.) And a toolkit with all the essentials (researched from the UKGSers forum – thanks chaps).
I’ve also replaced the alloys that are standard on the GS with a pair of spoked wheels from the GS Adventure. The theory is that you can fix spoked wheels if they get damaged but alloys are a write-off that will end the trip.