Former QPR assistant manager Kenny Jackett has been appointed manager of Portsmouth, replacing previous manager Paul Cook.
Cook left Fratton Park after being appointed the new manager at recently-relegated Wigan Athletic.
Jackett, who has managed Millwall, Wolves and Rotherham United in recent years, has been appointed on a two-year deal.
It is his first role since leaving Rotherham in November last year.
As part of his back room staff, Jackett has appointed former-QPR youth coach as his assistant manager.
The 44-year-old from Acton has worked as Jackett's number two at his last three clubs after crossing paths while in the QPR coaching set up.
Gallen enjoyed a successful playing career, playing for clubs including Watford, Exeter City and Shrewsbury Town, and representing the Republic of Ireland at Under-21 level.