Found everything I needed to know here : MAX BMW Motorcycles - BMW Parts & Technical Diagrams - R1200GS ADV 08-09 (K255)

The option was available up to 2008 on GSA. I'll check if the gearbox is the same as the 2009 GS and I might be able to do that. I also found that some police model have a 'walking pace' 1st gear.

 

I seem to recall some options on final drives too, ratio is stamped on them. Gearbox would be better for a wide ratio to help at low end and interstate speeds.

One of the benefits of chain drive.

 

Found another option, replacing the gearbox by one from a R1200RT-P (police version). They are remored to have a very low first speed for parades, allowing to follow a marching pace without slipping the clutch, it makes sense but have to be confirmed. I should also find if it's standard police issue or an option.

I have found one on eBay at a reasonable price. If I can confirm the information, I'll get it.

 

I'd love to know the answer to this question!

 

A call to your local friendly BMW shop should be able to confirm. I do know the GS/GSA ratios are different and the transmissions are a direct swap. Much easier than and probably more cost effective than an transplant.

If the R transmission works that good news too as the entire R bikes go for cheap. Buy it for the transmission and sell off the rest for parts.