This started as a reaction to a rant from one dairy lover who was offended by vegan cheese being called cheese. Excerpt from the rant suggesting the vegan cheese being called Gary:
"Call it Gary or something just don't call it
cheese because it's not cheese!"
The relevant part of the rant as an image is here.
The internet responded and so did the real world. Facebook page called 'It's not Vegan Cheese, it's Gary' has started and it collects humorous images and memes about vegan cheese being Gary.
Some supermarkets follow the trend as well - for example Sainsbury's (a grocery shop for those that are not from the UK) launched this ad on Facebook:
So vegan cheese is called Gary because of one particular non-vegan having first-world problems about the implication of it being cheese.