Oh my word I've just typed it into Google and there are a thousand and one articles about "Who is gammon? " and "Is gammon racist?".
I'd never even heard of it until a few minutes ago, am I getting old? Is that what happens when you're not in the Twittersphere?
Anyway it seems to come from somebody calling David Cameron 'gammon face' in relation to his ruddy complexion and as far as I can tell is mostly used by white people.
On that basis it doesn't sound particularly racist, and seeing as the other day the people who are complaining on this thread were talking about dropping Jamaicans out of aeroplanes to make 'Jamaican patties' I suspect they're not too bothered about avoiding offence and are just mischief making.