What is the Unique Identifier playerId in google-play-games?

Question

I want to use the unique username of a user logged in using Google Play Games V2 as an identifier on my database.

After researching, I found that the gamePlayerId in the Player resource seems to be the identifier, but the current return value does not output that value.

POST https://games.googleapis.com/games/v1/players/me

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I've also allowed next-gen player IDs in Google Play Console, but gamePlayerId is not exposed. Is this really an identifier? It just needs to be a unique value for each time the user logs in in that game. In that case, is it okay to use playerId?

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I check next-generator ID to use.