Which 80-year-old Gileadite had supplied David at Mahanaim but declined to go to Jerusalem?

Answer: Barzillai

Barzillai said he was too old to enjoy the king's court and asked David to take his servant Chimham instead.

“Now Barzillai the Gileadite had come down from Rogelim to cross the Jordan with the king and send him on his way from there.”— 2 Samuel 19:31 (BSB)