Starting in front: brakes, forks, wheels, headlight, speedometer,tachometer,dashboard, handle bars.
The oil cooler is in a different place and the exhaust is different.
The engine, transmission, reardrive are the same. BMW sold a kit to change a R100GS to a R100GSPD it was the big tank and the headlight crash bars. I saw a R100R with that on it, paint matched. it looked good.
The big difference is the front end.
Forks, I think you could get by with R100R forks but the rims are 2.50-18 the GS is 1.85-21. Tires, if you are going for a lot of dirt riding I have never seen 18" dual-sport tires. The rear rims are 2.50-17 on the R100R and the R100GS. You can find lots of tires road or off road.
Exhaust on the GS is high so it won't get damaged but that makes the left saddle bag smaller. If you mind the possable damage to the exhaust with the R100R you can have a larger laft bag.
Cost, I think if you want to go "off road" it would cost much more to change a R100R to a GS than to buy a GS and make changes to it.