An initial campaign for the price comparison site is expected to start over the next three months and further activity is planned for later in the year.
GT will also be responsible for a wider digital marketing strategy with Cheapflights.
The appointment of GT, following a competitive pitch against other as yet undisclosed agencies, completes the Cheapflights marketing roster.
UK marketing director Joe Sikorsky said: “We have recently restructured our marketing team, taking on three key members since March.
“The strengthened team will support our new and increased marketing activity and all of us look forward to working with GT.”
GT client services director Michelle Lonsdale said: “Cheapflights is a market defining company and the prospect of both determining and creating a new digital outlook for the brand is something that everyone within GT is thrilled to be working on”.
Cheapflights recently appointed Long Acre Partners to act as advisors prior to what could be a listing on the London Stock Exchange, sale of the business or injection of new capital to finance its aggressive growth strategy.