The point of the LG is to give something for SLS to do.

SLS isn't capable of sending all-up Apollo-style lunar mission with a crew module and a lander. So the lander needs to be sent separately. Therefore you need somewhere for the lander to dock, and since it's reusable, to refuel. Therefore, a Lunar Gateway.

If NASA had decided to build a rocket around a mission, instead of figuring out a mission to fit the rocket, then none of this would have been necessary.