Grayjay does not have sponsorblock. Grayjay does not sync my subscriptions across my other devices. Grayjay will not have YouTube features as they roll out. Etc.
Grayjay is basically newpipe (youtube scraper) + casting + centralized, unmoderated comments + other services.
YouTube Revanced is a patched, debloated version of YouTube itself.
Grayjay does a lot of cool stuff, but they are different tools for different problems. Youtube Revanced allows users to have a better Youtube experience on their phone without losing any functionality. Grayjay is asking users to change a lot about how they watch Youtube.