goalkeeper utilises indeed the same gun as the A-10. also, the USN has tested the goalkeeper, but they didn't wanted an 'foreign' system. so it is possible it could be placed on this ship, although it costs as much as 10 phalanxes (but IMO, it is worth the money
)
The other issue is that Phalanx would have to remain in the fleet regardless, as it has a much (
MUCH) lower footprint in weight and below-decks volume. Many USN ships with Phalanx could not accept Goalkeeper. And there is a huge drive for commonality - one wants as few systems in the fleet as possible. This means a common supply chain, common training requirements and so on.
Given the additional factor that CIWS is by nature a last-ditch weapon, and if it's firing you've already screwed the pooch to some degree, I think the USN made the right decision based on its own situation and requirements.