The agency picked up the business following chemistry meetings involving a number of undisclosed London agencies. The process was handled by the brand's founder, James Averdieck.
Gü, which plans to substantially boost its marketing spend in the next 12 months, has briefed Mother to create a communications strategy and above-the-line campaign.
Mother will work alongside Gü's design consultancy, Big Fish, to produce a campaign spanning traditional and non-traditional media, which will break in the autumn.
Since it launched in 2003, Gü, and its sister brand Frü, have become the third most popular chilled pot dessert brands on the market, behind Cadbury and Müller.
Averdieck said: "It was an easy decision. We needed a team that specialises in creating communication that is famous, popular and cultural. Gü has got so far without really participating in any mass communication, and that's what's called for now."