[Gixxer-18]

That washer and clip protect the oil seal from grit etc, and lock the shift shaft laterally. Too much movement of the shaft to the right can put pressure on the selector arm even disengaging it with the motor running, that could be bad news. 12mm external circlip from a local supplier and a washer large enough to cover the oil seal and you're back up and running.

 

[Gixxer-18]

Although the shaft is not in the engine; with external circlip pliers it's really easy to fit:

HTH.
Nigel.

 

[Gixxer-18]

Just have to remove the front sprocket cover, and gear lever to fit the washer and circlip. Clean the oil seal externally to make sure it's not leaking.. off you go again.
Nigel.