GOG will never be able to compete with Steam for one simple reason: DRM.
Steam is a safe and not very intrusive DRM system, but CD Projekt has one simple with rule with GOG, and that is no DRM. Consequently, third parties don't want to put their new games at GOG because they fear of piracy and, without them, they'll never be able to compete face to face with Steam.
Then in the end, people should try thinking that GoG>Steam, because it ultimately cannot compete with Steam due to their one policy they stick with.
That doesn't mean the other clients out there cannot compete with Steam, they are just lazy and only care about what they want to sell, not what consumers want.