I wasn't thinking return to Earth for Dragon LCOTS. If it could do that, then we could also just launch crew on Dragon instead of Orion. I doubt that the gateway will need downmass that Orion can't offer for a while.
Without the heatshield, parachutes, and superdracos, you would have an easier time getting there too.
(Tangent about how much cargo D2 could get to NRHO)
From what I can find, going from TLI to NRHO is around 450m/s (FH would get D2 to TLI just fine I think). I am having a very hard time finding a proper dry mass for dragon 2 and a proper fuel load... I am finding everything from 6.5 tons and 1.3 tons to 9.5 tons and 2.2 tons... I am going to go with the numbers on the environmental assessment for landing. https://www.faa.gov/about/office_org/headquarters_offices/ast/environmental/nepa_docs/review/launch/media/draft-ea_spacex-dragon-gulf-landing.pdf
Dry mass 7.7 tons, fuel mass up to 2.2 tons. Draco ISP of 300s. Delta V is about 740m/s. However, we have not factored in cargo. If we assume 100m/s for docking, undocking, and de-orbit plus safety margin, we need a total of 550. Running the numbers gives us, unfortunately, only 3 tons of cargo, half of what dragon is rated for. However, the heat shield, superdracos, and parachutes aren't light, so once we remove those we get maybe 3.5 tons or 4 if we're lucky. For comparison normal enhanced Cygnus can also carry 3.5-4 tons.
And that's off the shelf besides the removal of entry equipment and addition of comms/other lunar equipment.
I can't find any numbers on Cygnus but it's almost certain that it has less Delta-V than Dragon 2, as it wasn't designed with launch abort or propulsive landing in mind. It would take a lot more effort to modify Cygnus to be able to reach Gateway than it would Dragon 2.
Dragon 2 can also dock off the shelf, whereas Cygnus berths, and there won't be a robot arm on gateway for a while. This can be changed fairly easily compared to other things, but it's still a change.
If we're only thinking about minimal modifications to get going quickly with a low dev cost, Dragon 2 seems like a really good choice.
However... They already have to get a Cygnus Derivative to the gateway for the MHM/HALO module, so if they're already doing the development on getting something Cygnus-y to have the fuel, comms, docking, etc. to get to gateway, it would mean that they could do the same thing to a Cygnus if they play their cards right.
Which means that in this case, Cygnus is a really good choice because they already have to do big modifications to something closely related, including comms, which would still have to be modified on D2.
I wonder if they are considering just one provider or two... If two, then D2/Cygnus variants seem like good choices (one can get there almost off the shelf, one has the modifications already paid for). However, I doubt that Gateway will need two resupply carriers... I can't imagine needing more than one per crew, and that's one a year max.