Catlin, who was appointed as assistant to Alex Pike in 2009, announced his decision this morning on Twitter.
Catlin had been a player with the club before moving into management at Chichester City United, Petersfield Town, Portfield and Fleet Town.
Catlin, who was given the honour of leading out the Gosport team in the FA Trophy Final at Wembley in 2014 said on Twitter "I wish to announce that I have left @GosportBoroFC it was a difficult decision after 9 years but the right one for me & the club......
"I would like to thank @GosportBoroFC fans for their support & wish the club & the players past & present all the best for the future x."
- Catlin became the second established name to leave the club this week after Sam Pearce announced on Wednesday he would not be returning to the club when his loan at Farnborough ended.
Updated 10:36 - 13 Jan 2017 by Site Manager