I assume this is a gas piston upper and as long as a 7.62 AK bolt would fit the carrier and you could get a barrel head spaced for .125 inch bolt face depth I think it would work. I don’t know if there is a trunnion for mating the barrel and would think you’d need an AK pattern barrel. Honestly I have never looked at a Gallil Ace and like usual just speculating. I bet Krebs Custom or Sven at Manticore Arms would know have a good idea if a 6.5 Grendel Gallil Ace is feasible. I know the original Galil took design elements from the AK and used 5.56/223 or the large frame 7.62x51/308.
https://www.krebscustom.com/contact-us
https://manticorearms.com/contacts
https://www.krebscustom.com/contact-us
https://manticorearms.com/contacts