-
I use one of these jobbies from the JKC, £10 iirc. It's basically a rubber with imbedded grit. Works great on rust and stains but be a bit careful if you have a Kurouchi (rough iron clad) finish as it can remove some making it look uneven or likewise a mirror finished polished knife it will leave scratches if rubbed too hard.
Another tip I was given in a shop was to tear a dish cloth into a long 10cm strip, roll it up tight and gaffer tape the middle. Then when you're finished using a whetstone, mop up all the slurry goodness and use it to remove stains and rust in the future.
That's a sweet gyoto, nice gift! It's an unclad stainless steel so you won't have problems with rust but it's still best practise to regularly dry knives between use.