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'.
Notes
1
The Tookland was the ancestral folkland of the Tooks, and so Isumbras presumably had his seat there, probably in or near Tuckborough. We can be sure that, unlike his successors, Isumbras I did not occupy the great network of Hobbit-holes known as Great Smials. That residence did not exist in Isumbras' time, and was only begun by his descendant Isengrim II, who lived some three centuries later.
2
The pronunciation of Isumbras is difficult to discern, especially as the name combines elements from two different language families. The Isum- element is Germanic (and the initial 'I' sound would have been pronounced like the English word 'eye') while -bras comes from French (and would originally have pronounced with a silent 's', as something like '-brah').
However, as used by English speakers, the pronunciation of both these elements has evolved over time. Probably the most commonly used variant on Isum- is in the name 'Isambard', which conventionally uses a short 'i' sound (as in the word 'is'). The ending -bras is rare, but is best known from Fortinbras in Shakespeare's Hamlet, where the final 's' is pronounced rather than silent.
The original pronunciation of 'Isumbras', then, would have been something like '(eye)su'mbrah', but a more modern version would be 'isu'mbras' with a short 'i' sound. Either approach is defensible.
3
'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.