A person who frequently (usually at least once per week) plays video games.
If the person in question only ever plays one video game, that does not make him/her a gamer.
The word 'gamer' is vague, because it could refer to
people who play games as a sport or for a living. These are simply subdefinitions that
may be applicable to certain
people.
''
Nate's been playing Fallout 4 for months now, probably every day. He even got to level 100. Somehow, he's still got time to
play other games too.''
- ''Seriously? I didn't even know he's a gamer.
Maybe we should hang out sometime.''