R100GS Avon AM24 Gripster Tires
Avon's Gripster tire is old as dirt and I'd guess has been used on BMW's GS line almost since it's introduction. Designed from the git as a dual sport tire, the AM24 features front and rear specific tread designs that should be mounted following the directional arrows on the sidewall. Both tubeless and tube type varieties are available, though the tubless ones can be used with a tube. If you're riding an R100GS, you'll need a 90/90-21 for the front and a 130/80-17 out back.