Basically Gamergate started as some butthurt guy trying to get revenge on his ex by claiming she slept around to get better reviews on her (free) game. Gamergaters used this as an example of corruption, thus the rallying cry of "Ethics in gaming journalism".

However, in actuality Gamergate was driven by gaming going from a niche hobby to a mainstream entertainment. Many gamers were increasingly unhappy that games stopped exclusively catering to people like them (i.e. white and nerdy males) and started making token efforts to appeal to other groups. This combined with the spark of the butthurt guy's lame attempt at slander birthed Gamergate, a fig leaf of "ethics" used to justify and to cover up a seething mass of resentment, misogyny and racism.