We don't know how this new Thain was installed, though the original choice of Bucca apparently involved an election, so Gorhendad's replacement was perhaps installed by a similar process. The new Shire-thain, the thirteenth to hold the office, was Isumbras, a member of the Took clan. Already an important family within the Shire, the installation of Isumbras established the Tooks as the source of the hereditary Thains of the Shire down to the War of the Ring (about seven hundred years later) and beyond. Isumbras was followed by more than twenty Thains who could claim him as an ancestor, including at least three who also took the name 'Isumbras'. For that reason, Isumbras the successor to Gorhendad Oldbuck and first Thain of the Took line is known to history as 'Isumbras Took the first'.
'Iron arm' is the literal meaning of the name, but Tolkien's choice here was likely influenced by Sir Isumbras (or Isembras), the pious hero of a medieval romance.