First, Galaxy Fist fits Garp better.
However the point of that scene is to showcase the magnitude of Top Tier level attack. So, "bigger sounding names" wins, thus, Galaxy Impact.
Secondly, there are actually third option which I like better, Divine Knights.
This name is often used in some other media, however it's rarely used in One Piece world terminologies. Holy Knights looks like fits better for the noble/royal image the CD shows.
But, again, if the intended goal is to "sound bigger" like Galaxy Impact, and less about relevancy, God's Knight should be used.