No, keybound spirits won't be able to join, not celestial wizards. Meaning Gao wouldn't be able to be in it.

And Lucy used her spirits because spirits weren't allowed in the Grand Magic Games. The problem isn't that Celestial Wizards are overpowered (because they really aren't), it's because if one of Eli's spirits were in it, then he wouldn't be able to use them, as they would count as an enemy.