Is there a way to get the Google Play ID (number, line, whatever) from a specific user when using Google Play services?
For Google Play gaming services, you can use:
String playerId = Games.Players.getCurrentPlayerId(getApiClient());
It depends on the service you are trying to access. Here is the corresponding Player class for Google Play games: https://developer.android.com/reference/com/google/android/gms/games/Player.html
Player
And the corresponding People class from Google Plus: https://developer.android.com/reference/com/google/android/gms/plus/model/people/package-summary.html
People
Finally, there is AdvertisingIdClient for the Google Mobile Ads platform: https://developer.android.com/reference/com/google/android/gms/ads/identifier/AdvertisingIdClient.html
AdvertisingIdClient