With peat sales under pressure from Defra, the GMI is designed to show that the industry is serious about reduction.
But so far no garden centres have signed up, despite B&Q, Homebase, Scotts, Westland, the RSPB, Defra and the RHS being members of the scheme.
Dobbies and Ayletts Garden Centres are working towards membership.
Gwyther said Defra secretary of state Hilary Benn praised the GMI in a meeting last week but added: "We have to encourage more retailers to join or it will fall down."
GMI manager Tim Briercliffe commented: "The GMI hinges on garden retailers being members. We have all the other sectors represented including key DIY retailers like Homebase and B&Q."
He added: "There will be an increased need for retailers to provide information and support for consumers in buying reduced peat products. Getting involved in the GMI is a way of demonstrating that. Responsible retailers should take this seriously. They don't want to be a target of environmental campaigners.
"[GMI members] Scotts and Westland are labelling bags with their peat content. Although we're moving towards standard labelling its not going to be on every bag next year."
The Government is planning an Act on CO2 campaign intended to engage with retailers. It is due to launch in February 2010.
- The Know Your Compost campaign from the Government's Recycle Now initiative is encouraging garden centre, DIY and supermarket managers to use posters and shelf wobblers to promote peat-free products in store. The materials, which feature the slogan "A greener choice", are available to download from www.recyclenowpartners.org.uk.