There are definitely games where the primary/target demographic is boys or girls though.

The arcade classic "Centipede" is a good example of this. It was designed to try to attract women to play it. The colors were pastel shades and its sequel Millipede took it further by having a "story" about an elf archer protecting a mushroom forest from a bug invasion. Centipede was partially programmed by a woman (one of four people who made the game), Donna Bailey, who was also responsible for choosing the games vibrant pastel color palette.

Now, is Centipede or Millipede only for girls? No of course not. No video game is only for a single demographic. But real world data showed that girls/ women generally played Centipede and Millipede more than boys/men did. Some things just have a general appeal to some demographics more than others. So typically when a person asks for "girl games," they just means games that will have a high appeal to girls or games that were designed with girls as the primary demographic.

Otome visual novels are this way as well. They target a female playerbase with a story-based romance game and generally feature a female protagonist who romances male characters in the game.