The fork system on the 650gs is a simple plunger or damping rod design and it could be considered a weak point on an otherwise capable machine. The suspension is also biased towards road use rather than dirt use. The choices to improve the performance are complete replacement of the forks or fitting emulators to bring the standard showa fork performance up towards modern cartridge fork capabilities.
Careful consideration should be given to the usage of the machine before undertaking any upgrade.
The lowest cost upgrade is fitting emulators and would suit road usage. YZ or WP conversions provide increased travel and would suit those looking for more off road oriented suspension.
Emulators are available from a number of suppliers.
They make damper rod forks more like cartridge forks.
Emulators are adjustable valves which are installed on top of the damper rods and separate the compression and rebound fluid pathways. They are held in place by the springs.
Install is relatively simple and can be tuned for rider and machine usage.
Ricor units do not require modification of the damper rods unlike Racetech.
There is a 0.5mm difference in staunchion tube thickness between GS and Dakar models, when fitting emulators ensure the correct diameter seal is used or the emulators will not provide the improvements to be expected.
Also see the spring rate graph below, emulators make greater use of damping to conrol fork movement so lighter spring rates are used as per cartridge forks. This means fitting emulators reduces any need to increase spring rate.
This is the easiest of the fork replacement options, it is well discussed on advrider and parts for retention of the BMW front wheel, disc and caliper can be purchased. The 46mm forks are the simplest option.
The most difficult part is construction of the ignition switch bracket.
House of Horsepower can shorten, revalve and respring the forks to suit the GS. Any tuner should be able to revalve the forks for the GS.
Advrider forum inmate Aquatic has Axle Kits.
HOH and Aquatic kits are for open chamber forks. See YZ Reference
This is the more complex replacement option, WP 4357 or 4860 forks can be used. The discontinued Touratech kit used 4357 WP forks.
An axle needs to be manufactured to retain the BMW front wheel but the BMW caliper can be retained using a KTM front wheel, KTM 300mm disc and WP caliper bracket. The KTM/WP caliper bracket is a Brembo part.
There is a larger range of springs for the 4860's but the larger diameter of the forks limits steering lock.
There have been other fork conversions done.
They are not as well documented as the WP/YZ forks
A write up on a DRZ 400 fork conversion DRZ 400 pdf
The basic fork lengths, travel and weights of the G650X and Husky Terra for comparison G650X-Terra pdf