The outlet recently released its 'Jack-in-a-bun' vegan jackfruit burger featuring a butternut squash and quinoa patty, beetroot mayo Korean pulled jackfruit, pickled onions and rocket in a sourdough bun.
GBK will be giving away 10 free burgers in each of its restaurants across the UK. Diners must say 'Jack on the house' at the till to claim theirs. Free burgers are on a first come, first serve basis, one per customer.
"We want everyone to be able to enjoy incredible burgers at GBK, meat-eaters, veggies and vegans alike," Jay James, Head of Marketing at GBK, said in a statement sent to Plant Based News.
"With our giveaway on Thursday, we want burger-lovers to unite and head down to our restaurants to enjoy a ‘jack-in-a-bun-on-the-house!"
The Jack-in-a-bun is available at GBK restaurants nationwide for £8.95.