When referring to multiplayer games I see a good gamer as someone who has the ability to outperform at least 50% of the playerbase. But also they are helpful to newer players and help to teach strategies and concepts to help their teammates who know less. I don’t think that the people with the most kills that are berating everyone are necessarily good. I think they are skillful, but they can’t help their team as much as if they were trying to support the lower skilled portion of it. I am not perfect either, but if a newer player on my team is complaining that they don’t know what is happening or even on the enemy team, I’ll try and help as much as I can.
In single player games I really don’t think there is a “good” gamer. Single player is just that, for you to play alone. If you enjoy slow, and careful gameplay, then you are good if you play however you want. I really don’t think you can measure how “good” someone is in a environment where everyone is playing how they want to. In single player environments I try to make everything as slow, large scale, and epic as possible. That means in games like Star Craft, I’d rather let the enemy have time to build up so we can have a titanic clash of huge proportions. I don’t like speed running things and doing the minimum to win. However the people who enjoy that are certainly able to play how they like.