If you have to conduct adjustments to your BMW R 80 GS then it is usually a great idea to own a Workshop Manual to use as a reference in the event that you want to know tips to strip off a particular part for example, the Gearbox and install it again properly.
BMW R 80 GS Workshop Manuals can offer you particular diagnostic details for the vehicle so if something is broken it will help you determine what the issue is and tips on how to resolve it. After that you can decide if this can be achieved yourself or in case the job is too big you could make a reservation for your R 80 GS in with a local mechanic.
BMW R 80 GS manual