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NASA is looking at domestic commercial launchers powerful enough to reach lunar orbit for non-crewed elements of Gateway. As far as we know now of what will be available at the intended launch time in 2024, that will be SpaceX Falcon Heavy, ULA Vulcan, or Blue Origin New Glenn.

Falcon Heavy has the advantage that it's already in service. The other two are currently expected to make their maiden flights late this year.

It's possible that SpaceX could get Starship ready for launch by 2024 too, at their aggressive rate of development, which would be super cool, though it's early to order for launch services.