I noticed that the light button on my GW-5000 was sticking a bit which was making it hard to consistently get it to activate. I know the GW-5000 is widely thought to have hard to press buttons but this was unusually difficult. I figured it was probably dirt or grime as I wear the GW-5000 frequently, including at the gym and in the shower.
So I removed the bezel and had a look. Once the bezel was off I could clearly see that the button was sticking; it would not readily return to the rest position. I started by using a soft toothbrush around the button to try to clean any dirt or debris away from the crevice. This helped a bit. I then put a drop of air tool oil on the button and let it sit for a while. Once it "soaked in" I started pressing the button repeatedly (a couple hundred times) and then spun the button back and forth for 10-15 minutes. Success! It completely cleared up the sticking issue.
I repeated this with the other buttons and had similar results. I am not claiming that this is a fix for the stiff buttons, but I think lubricating and wearing-in the seals would logically help button movement if it is starting to degrade.
My ideal method would be to remove the buttons from the inside and clean/lubricate them directly, and I have done this on other watches, but I don't want to open my GW-5000 unless I absolutely have to (no need for trimming or battery changes, at least not in the foreseeable future).