The ads, which appeared on the London Underground on 16 January, featured a picture of a hamburger with the caption: "Vegetarians, resistance is futile"; and a picture of a GBK receipt alongside the caption: "You’ll always remember when you gave up being a vegetarian".
Another ad showed a picture of a cow, which was captioned: "They eat grass so you don’t have to".
As of 4pm today, the Advertising Standards Authority had received 150 complaints about the ads.
A spokeswoman for the ASA said the watchdog was carefully assessing the complaints to establish if they are grounds for further action.
After a backlash on social media, the brand announced that it was "taken aback" by the reaction and would remove some of the ads.
In a statement on its Twitter feed, GBK said: "The last thing we ever intended to do was offend or alienate vegetarians. The same vegetarians we've looked after since our first GBK.
"Our intentions were light-hearted and not meant to cause any offence, but clearly we have, and for that we apologise.
"While we've served beef at the core of our menu since 2001, we've always catered well to the veggies out there, and always will."
The Vegan Society said GBK appeared to be unaware of the legal status of Veganism as a protected belief under equality laws.
Jasmijn de Boo, the chief executive of The Vegan Society, said: "What a wonderful example of the power of the people to change things. They spotted an injustice, and made their voices heard. GBK had no choice but to listen and react.
"GBK has shown itself to be totally out of touch. These outdated and highly offensive ads were completely misjudged and smacked of both naivety and ignorance. Were they not aware of the growth of the vegan movement?"