When you enter Game Settings in the Options menu there's an FPS setting that works close enough to GameSpeed. Change it by intervals of 5 FPS to get the style of classic MUGEN GameSpeed. If you want to edit it in Config.json edit Gameframerate and not Framerate. The latter is used solely for animation interpolation while Gameframerate is the Gamespeed equivalent.
Hopefully someday I'll be able to make the little puffball on my profile pic.
(Sprite edited from Scrollboss' sprite)