The guild, a consortium of business travel agents, handed the brief to the agency following a competitive pitch.
Campaigners are calling for green taxation to be increased and the Government could raise taxes such as air passenger duty.
Cavendish account director Gareth Morgan said: 'The GTMC is asking us to advise on issues that affect its members, and business travellers, including airport capacity and the environmental debate around air travel.'
Cavendish will work alongside Brighter PR, the GTMC's retained agency.
