I've had this on my GSXS (Its a fairly common fault)
First time I cleaned it and it helped but only for a few months, next time I replaced it and its fine now.
I've started a write up on it here
https://gsx-s.info/suzuki-gear-position-sensor/ not much writing yet but plenty of pictures
It is behind the sprocket cover, just need to release the clutch cable to remove the cover - GPS comes off very easy but a little fiddly to replace if you have big hands like me - just takes a bit of practice
Part No. is 37730-41G02