The 30 year old Gore born player will move to England to link up with the Cherry and Whites once his commitments in New Zealand have come to an end.
At international level, he has won more than 50 caps for New Zealand and was part of the 2011 Rugby World Cup squad, making four appearances in the tournament.
30 year old Cowan has been a regular at Super Rugby Level with the Highlanders since 2003 and a member of the All Black squad since 2004.
He has long been regarded as an influential figure in the Highlanders side and was named captain in 2009.
He made his 100th Super Rugby appearance for the Highlanders against the Stormers recently, a feat only previously achieved by Anton Oliver.
He plays his provincial rugby for the Southland Stags and will be hoping to pass 100 appearances for the Stags before he joins Gloucester, in the process achieving a notable 50/ 100/ 100 milestone.