Actually, I’m using Google mail with less secure apps setup in java webapp.
Google decided to deprecate this functionality and I have to migrate to use Google API to send mails through Oauth2
I tried several options, including the Google documentation but with no luck. I’m very stressed because I need it solved before Google shut down less secure apps compatibility.

I have a Google Workspace Account for non-profit, which means that I have full access to all Suite.
I created a Full Access Service Account.
I enabled Google Gmail API.
I created an OAuth 2.0 Client ID with an authorized URL redirection to http://localhost:8080/ROOT/oauth2callback
I created an OAuth consent screen.

This is my java code:

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.security.GeneralSecurityException;
import java.util.Collections;
import java.util.List;

import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp;
import com.google.api.client.extensions.jetty.auth.oauth2.LocalServerReceiver;
import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow;
import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.gson.GsonFactory;
import com.google.api.client.util.store.FileDataStoreFactory;
import com.google.api.services.gmail.Gmail;
import com.google.api.services.gmail.GmailScopes;
import com.google.api.services.gmail.model.Label;
import com.google.api.services.gmail.model.ListLabelsResponse;

/* class to demonstrate use of Gmail list labels API */
public class GmailQuickstart {
  /**
   * Application name.
   */
  private static final String APPLICATION_NAME = "Gmail API Java Quickstart";
  /**
   * Global instance of the JSON factory.
   */
  private static final JsonFactory JSON_FACTORY = GsonFactory.getDefaultInstance();
  /**
   * Directory to store authorization tokens for this application.
   */
  private static final String TOKENS_DIRECTORY_PATH = "tokens";

  /**
   * Global instance of the scopes required by this quickstart.
   * If modifying these scopes, delete your previously saved tokens/ folder.
   */
  private static final List<String> SCOPES = Collections.singletonList(GmailScopes.GMAIL_LABELS);

  /**
   * Creates an authorized Credential object.
   *
   * @param HTTP_TRANSPORT The network HTTP Transport.
   * @return An authorized Credential object.
   * @throws IOException If the credentials.json file cannot be found.
   */
  private static Credential getCredentials(final NetHttpTransport HTTP_TRANSPORT)
      throws IOException {
    // Load client secrets.
      File file = new File("/a/b/tokens/eac-webapp-client_secret.json");
      FileInputStream credentialsStream = new FileInputStream(file);
    GoogleClientSecrets clientSecrets =
        GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(credentialsStream));

    // Build flow and trigger user authorization request.
    GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
        HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, SCOPES)
        .setDataStoreFactory(new FileDataStoreFactory(new java.io.File(TOKENS_DIRECTORY_PATH)))
        .setAccessType("offline")
        .build();
    LocalServerReceiver receiver = new LocalServerReceiver.Builder().setPort(43513).build();
    Credential credential = new AuthorizationCodeInstalledApp(flow, receiver).authorize("user");
    //returns an authorized Credential object.
    return credential;
  }

  public static void main(String... args) throws IOException, GeneralSecurityException {
    // Build a new authorized API client service.
    final NetHttpTransport HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
    Gmail service = new Gmail.Builder(HTTP_TRANSPORT, JSON_FACTORY, getCredentials(HTTP_TRANSPORT))
        .setApplicationName(APPLICATION_NAME)
        .build();

    // Print the labels in the user's account.
    String user = "me";
    ListLabelsResponse listResponse = service.users().labels().list(user).execute();
    List<Label> labels = listResponse.getLabels();
    if (labels.isEmpty()) {
      System.out.println("No labels found.");
    } else {
      System.out.println("Labels:");
      for (Label label : labels) {
        System.out.printf("- %sn", label.getName());
      }
    }
  }
}

The file I’m using to create the GoogleClientSecrets is the one I get from OAuth 2.0 Client ID.

But always, at the moment when executing authorize method, the received message when opens in browser is Access blocked: This app’s request is invalid (Check de screenshot)

And I’m blocked at this point.
Has anybody an idea to solve this situation?

UPDATE:
I’m trying to understand why google is giving me this information:
request details: redirect_uri=http://localhost:8081/Callback flowName=GeneralOAuthFlow

What is this URL? What is intended for?, I have to develop something in there?

Thank you all.

g2h2o