As of snapshot 17w49b, the 'gameLoopFunction' gamerule is missing

I was updating my functions from 1.12 to the 1.13 snapshot 17w50a and in this version i found that the gameLoopFunction gamerule is missing I looked to see if it was moved to it own command but i did not see anything that looked like it would be it. and when i try the gamerule it just says "Incorrect argument for command at position 9: gamerule <--[HERE]" please fix this i don't always like using commands to activate my main function. thanks.