Gil is the frog attendant, Nagi's entourage. Retainers keep the same lifespan as their masters, but that does not apply to their partners. Meaning that Gil will live as long as Nagi will, but not his spouses who have a standard lifespan. So they will keep getting older and eventually dying, while Gil remains alive T_T.
Well, it hasn't been said, but... considering that Gil has already widowed five times, it's safe to assume that no, there's no way around that. Or else they would have already done it. Even if you're a sacred beast, you cannot tamper freely with the life and death of other beings, as Kureha told Kiyomori.
I WANT MORE HUMAN MR FROG.. If i remember, mr frog alreadh have a partner right?? Can we have his backstory (〜 ̄△ ̄)〜