It all depends how much control Kubik has with the Reality gem... But I see Galactus winning most battles. Normally Kubik considered to be no match to a Celestial so a well fed Galactus should have no problem with a cube but the R gem can give Kubik a chance just depends what he can do with it.
And the gems separate are nothing amazing and pull potential have yet to be seen.
It has been shown that the more advanced the user was in reality manipulation, the more extent they could use an infinity gem.
Kubik has been manipulating reality from the get-go, the reality gem alone could extend his control to universal. Kubik has fought against other reality manipulators and won without the gem.