The agency won the business, worth an estimated £4 million, following a competitive pitch against a number of undisclosed agencies.
Its appointment marks the first time GWR has centralised its media brief into one shop. GWR had previously used agencies on an ad hoc basis, with Mediaedge:cia having handled media activity for the Classic FM brand most recently.
Rocket's assignment will comprise activity across all aspects of GWR's radio interests, including its flagship Classic FM station and associated brand spin-offs, such as CDs and The Mix network of local radio stations.
As well as providing and implementing media strategy, Rocket will work alongside Opus, GWR's sales operation, to provide media intelligence and market insight.
Rocket will mainly be working on the largest stations within GWR's The Mix network, including Trent FM in Nottingham, Essex FM and GWR FM in Bristol.
Classic FM is the UK's largest commercial radio station with a national reach of 6.7 million adults per week. The latest Rajars reveal that the station grew its listening share to 4.7 per cent.
Colin Dudgeon, the marketing director of GWR's local radio group, said: "Rocket is an agency with precisely the sort of fresh, innovative and insightful approach GWR will benefit from."