So typically most of the metal and platic parts can be cleaned with dish washing soap. For the more problematic stains that can’t be wiped off with this I use “Schmutzradierer” (“dirt eraser”). I do not use it on bare metal but coated ones. It’s basically a melamine resin foam based sponge. Even though it has a basic toxicity when absorbed by the body, in mainland europe it’s still a quite commen household cleansing item. Sensitive surfices may get damaged if you apply too much pressure.
Since this sponge usually cleans mechanically instead of chemically I guess you shouldn’t use it on the most expensive GMK set you have. But you know how it goes… 
I don’t care about plastics 
For remains of adhesives I use acetone. But then again this is also not the healthiest stuff to work with. But this is laso dangerous on PCBs because you knever know with what kind of compound the printed wiring is protected against liquids or acids. So be warned.
Unlike my 5 bucks membrane boards which I usually given to recycling after they were clogged up with dirt beyond economical repair. I’ll remove they keycaps every month to clean them and the plate. As my RK87 has no flex cuts in the plate and the LEDs are hidden between a layer of PE foam between plate and pcb the pcb doesn’t get too dirty if you don’t over-lube your stabs.
The LED cutouts in the switches are the main problem. But I have no other solution then to blow them out with canned, pressurized air.
I’ve also tried to remove G-Lube and Krytox 205 with dish washing soap and a brush/scrubber? from coated metal plates but with mixed results. Switch and stab lubes are just so much more potent than kebap sauce on a plate. 
Even though the result of the failed lube cleaning on a metal plate could only be seen with a spray paint job applied. All stained parts (mostly around over lubed stabilizer wires) heavily failed in accepting the spray paint. Without such an use case I think dish washer soap is ok since dirts didn’t accumulate on the stains where the lube was more than on other parts of coated plates.