(a guide by rika (@lostkagamine). tested, should work fine.)
If you are having issues like the screen turning black, read the mini-FAQ at the bottom of this document!
Uninstalling GShade removes the gshade-shaders folder which contains all the content. It does not remove GShade's executable file. This guide will help you do that.
If you already uninstalled it, reinstall it. Sounds weird, I know, but just don't open the game.