Even though Garlic is immortal is is heavily outmatched here. I think Garlic would get beaten unconscious and BFR'd. So this is pretty much between Slug and Frieza, and I believe Frieza can win here with massive difficulty.
Actually my friend, Lord Slug should be around second form Frieza's at best(that's me being nice) I really think first form. I remember King Kai saying strong as Frieza but third form wasn't even revealed yet in the canon. But if its that Vegeta then Goku is gonna get pwned.