Formed in 2011, GETA is dedicated to promoting gay travel into and within Europe, a market estimated to be worth €50bn (£39.6bn) per year.
As well as promoting the work of GETA globally, Grayling will also support Gaywelcomms, a full PR, advertising and social media service for businesses wanting to reach LGBT tourists.
The contract with Grayling also includes GayWelcome, which showcases European events, hotels, tours and cruises, and contains up to date travel news and destination guides.
Carlos Kytka is executive director at GETA, which has previously worked with Rain Communications.
He said: "It isn’t about painting your hotel pink and waving a few rainbow flags; it’s about making small changes to established processes and services to make gay people feel truly welcome.
"We have a number of campaigns planned for the coming months, with some exciting partners. Our aim is to make travel in Europe a diverse and inclusive experience and to help our members provide, and the LGBT public enjoy, a great travel experience."
GETA aims to help its 4,200 corporate members understand the scale and characteristics of the European gay tourism market.