There were official procedures for everything when it came to receiving a noble title.
It wasn't as simple as Desmond announcing it as such, and it is so. The duke had to receive special permission to appoint a baron on behalf of the king. For months, he sent message after message singing Gerard's praises until Stephano deemed him worthy enough.
After Gerard accepted the responsibilities, they would have to enter the Durands in a registry in Coeurvalle that would give them special permissions and name an heir for the land so it would never be without a leader.
With the title, the Durands would finally have the ability to formally recruit a military force that would be at Gerard's will rather than the duke's. Even though he would have to consider the well-being of greater Montclair in his decisions, he wouldn't have to always wait for Desmond's response–which was especially hard to come by due to the very nature of secluded Montville.