The rockets are fine, but they had better do some serious research to make sure the Gateway can handle all kinds of space radiation and other dangers (from all over the electro-magnetic spectrum) of a _long-term_ orbiting system around the moon.
There will not be _any_ atmosphere to protect it, so this is _totally_ untested technology.
I doubt if there is much difference between the Gateway orbiting the Moon and the current International Space Station that has been orbiting the earth for a very long time as far as radiation etc are concerned.