Threepipe was awarded the account, previously held by Redleaf Communications, without a competitive pitch.
The chain is embarking on a drive to grow market share and Threepipe will handle PR for new menu launches and restaurant openings. Threepipe will also support the firm's loyalty programme, GBK Brand, which was launched earlier this year.
Threepipe co-founder Jim Hawker will lead the account, reporting to David Sykes at Clapham House Group, which owns the GBK brand.
'We will be working closely with GBK to communicate all the innovation and to distinguish it from the wide high- street choice,' said Hawker. 'We will be helping GBK communicate the strength of the brand at a time when many restaurants are closing.'
GBK recently launched its 50th restaurant on Clink Street in London and Threepipe will be helping to promote it.
GBK is set to launch a new style of restaurant featuring smaller-sized burgers in an effort to encourage greater frequency of visits from its customers.