Gallen, who has a long-standing relationship with Jackett, will reunite with him for to work for a fifth time – with the pair most recently together at Portsmouth.
He first joined up with Jackett at Milwall in 2013, having worked together previously at QPR, and has since been his trusted no.2.
“I’m very pleased to be here, and it’s a fantastic opportunity for me to come in with Kenny. On the back of a solid season for the club last year, we’re looking to improve upon that,” said Gallen.
“Kenny is a highly experienced manager, and I’ve worked with him on a number of occasions. We have a clear working relationship where I know exactly what he wants and requires from the players, and I’m the link between him and the players.”
“I know exactly what he wants and requires from the players.”
“I’ll be closer to the players on a daily basis, and my role on job is to coach the team and work with individuals to improve them. My skills are on the training ground.”
“There’s a good basis to work from. Players have left, and with that is a good amount of space – we need to recruit the right type of player and person, to push the club onto the top end of League Two.”
“He’s very strong on the training ground and is a great complement to myself. He’s very good with the players, and getting the best out of them, which is a big thing,” said Manager Kenny Jackett .
“He’s vastly experienced, and the backroom staff is a vital cog – it gives the club, and the team, stability and structure. Things that you need to build a platform on.”
The appointment will therefore see Danny Senda depart the club from his role as Assistant Head Coach, following his appointment last summer.
The club would like to place on record our thanks to Danny for his hard work, and wish him all the best in the future.