Gray added: "He is good in the air and brings a lot of power to our left-hand side. He has a good fitness record and that is also important.
"He is also a big influence in the dressing room and is an excellent example for the young players to look up to."
Holt is the second out-of-contract player to sign new terms with the club recently, following on from midfielder Ryan Gilligan.
"I am pleased that we have got two of the three out-of-contract players signed up early," said Gray.
"They could have kept their options open for another month or so but to have them sign so early shows they have confidence in the club and what we are trying to do and that they are happy here.
"Removing that uncertainty early helps me plan and lets the players enjoy their summer knowing that their future is sorted. "
Northampton's only out-of-contract player yet to agree terms is striker Adebayo Akinfenwa.