Long, who was capped twice by England, has had previous spells at Bath, Munster, Rotherham and Newcastle Falcons before moving to Northampton in 2009.
The 34-year-old will now compete with Huia Edmonds and Darren Dawidiuk for the No.2 jersey at Kingsholm.
"I've been fortunate enough to play for some great clubs in my time, but I have to admit that I'm really looking forward to joining Gloucester and playing in front of one of the most passionate crowds in rugby," he told the club's official website.
"I've always relished playing there and have experienced some great battles in front of the Shed.
"Gloucester have made some great signings for next season and I'm looking forward to playing my part in what should be an exciting season.
"In the meantime, however, I've still got work to do for the Saints and am 100 per cent committed to that. The club have been great to me and I've enjoyed my time there."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
$29.99
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
