What rating do you need to have to get a GM rank?

It's just interesting because some people with a bigger rating than others don't have a GM rank

So how do you get it?

An FIDE over-the-board classical rating of 2500 is one of the conditions needed, amongst others, for application to become GM.

To become a grandmaster, a player must achieve both of the following:

• An Elo rating of at least 2500 at any point in their career (although they need not maintain this level to obtain or keep the title).
  • The rating requirement can be fulfilled even if the player starts a tournament rated below 2500 and then reaches or exceeds a 2500 rating during the tournament but eventually concludes the tournament with a rating lower than 2500.
• Two favorable results (called norms) from a total of at least 27 games in tournaments. With some exceptions, to receive a norm in a tournament:
  • The player's rating performance at the end of the tournament must be at least 2600. (Tournaments are no longer classified in categories.)
  • At least 33% of the player's opponents must be Grandmasters.
  • At least 50% of the player's opponents must hold a FIDE title.
  • The player's opponents must have an average rating of at least 2380.
  • The player's opponents must come from at least 3 different chess federations, which can include the player's own federation.
    • A maximum of 60% of a player's opponents can come from the player's own federation.
    • A maximum of 66% of a player's opponents can come from a single federation.

The Grandmaster title is also automatically conferred, without needing to fulfill the above criteria, when winning the Women's World Championship, the World Junior Championship, or the World Senior Championship.

Source: FIDE Handbook

#2 This happens because:

• Even if your rating goes down after you earn the title, it doesn't get "revoked".
• Some people have a high rating but no norms.

Rating means FIDE rating, not chess.com rating, btw.