Gareth made a Wales try scoring debut against Italy in 2001 and was first choice for the Wales team during the 2003 Rugby World Cup. He also featured in the 2007 tournament and starred in 2008 victory over Australia in 2008.
The 46 times capped scrumhalf, who picked up Grand Slam winning medals in 2005 and 2008, also toured with the British and Irish Lions to New Zealand in 2005.
Speaking of his departure Gareth said;
"I've come to an amicable agreement with Cardiff Blues and would like to thank them for all their support over the years. It's been a frustrating couple of years for me and things haven't quite worked as I'd hoped. I don't think I've shown my full potential at the Blues due to the groin injury.
I've been told by the medical team that I need a full rest from the game for between 6 to 12 months. The groin injury started to hurt in the summer of 2009 and it's got progressively worse. Throughout that period I'd been playing with a lot of pain and had to have two operations on the groin and then a double hernia operation.
This season I've played 20 minutes against Treviso and started against Connacht but I had to take pain killers, so the doctors have recommended that I am to rest fully for up to a year.
Obviously I will miss all the players and management who have been fantastic with me since I arrived and would like to wish them the best of luck for the future."
Speaking of the announcement, Cardiff Blues Chief Executive Robert Norster said;
"It is hugely disappointing news for both Gareth and the Blues that such a talented player is sidelined in this way. Gareth has overcome a number of serious injuries over the course of his impressive career, but this most recent injury has really caused tremendous frustration not to mention significant pain for the player that prevents his successful return to competitive action. Naturally, on behalf of everyone associated with the Blues I thank Gareth for his efforts for the team and wish him every success in his future career".