BMW introduced R100GS (for Gelände Sport, Gelände meaning something between "country" and "field" in German) in 1988, and the Paris-Dakar version in 1990. Starting in 1992, the Paris-Dakar was re-designated the PD due to some legal conflict with the organizers of the Paris-Dakar rally, for which it was named.
The GS is introduced, replacing the G/S. The US got only the R100GS, but an R80GS was sold in other markets. The major differences between the R80GS and R100GS are:
the oil-cooler on the R100GS
a small 'tombstone' windshield on the R100GS
798cc vs. 980cc. The R100GS got 40mm carbs in non-US markets while the rest got 32mm carbs.
Symmetric rear brake cam and shoes (was asymmetric)
Improved rear brake lever (according to press release)
At first the components were only available as a kit, but for the 1990 model year the Paris-Dakar was introduced.
Nylon tank (9+ gallons) with a locking compartment
Bigger oilpan bash plate and centerstand bash plate
Plastic front fairing for the lower engine (the 'bra') mounted to the crash bars
Dash board with the speedometer and control lights of the previous version and small tachometer and clock
Solo seat with "solo rack" that stayed on the subframe when the solo seat was removed
Frame-mounted 1/2 fairing with rectangular headlight, windshield and turn signals
When the solo rack is retrofitted, the seat latch plate (with passenger grab handle) is removed from the subframe and installed on the front of the solo rack. PDs which shipped with the solo seat (not all did) had latch plates without a grab handle.
1991-on GS and PD
In 1991 the GS got the PD-style fairing, shaped differently to match the steel tank. The dashboard is altered for both models, now having a large speedometer and tachometer. The clock is available as an accessory that bolts onto the handlebar clamps (and looks terrible). The other major change is the switchgear; these bikes now have the pushbutton switches that were introduced on the K series. Other changes:
Completely revised wiring harness
Floating front brake rotor
Stainless muffler (with separate brackets) replaces the original chromed steel one.
Revised bash plate with a redesigned oil pan having the drain in the rear wall.