I understand the good intentions behind this ad campaign, but they've gone and shot themselves in the foot. It's similar to telling someone, "Don't say 'gay', retard!"
The term "gamer" carries a lot of negative baggage, though not quite as much as the words associated with LGBTs. Obesity, incompetence, violent behavior and crime... all stigmas that are associated with video games and those that enjoy them.
Yes, the slogan in the ad hits home for a lot of people. There's truth in what they're saying, but not a lot of tact. It takes advantage of a hurtful stereotype to provoke thought about another one.
Ultimately, it's a nearly useless tactic. Those with the brains to actually consider the message are probably not the perpetrators of the slur. Make no mistake, we're talking mostly about young people here. They don't change their behavior overnight because of a thought-provoking poster. In fact, they're more likely to ridicule it on the Internet and go on for pages and pages about it in forums.
The target of this advertising should be parents. Who else shapes the world view and behavior of children so significantly? And while many will still use slurs like "gay" throughout their childhood, one hopes that parental guidance will teach them to abandon this bigotry as they mature.
At the very least, kids will learn to suppress their bigoted outbursts and keep their thoughts to themselves. No one is free of prejudice, myself included. I may view the majority of the 6 billion people on this planet as scum, but it doesn't do me any good to go around pointing it out. After all, there are a lot more of them then there is of me.