[SOLVED] How to get access token after user is signed in from Gmail in Android?

For your requirements, you can use the following code:

Firstly, make sure you have a valid Web OAuth 2.0 Client ID:

<!-- Server Client ID.  This should be a valid Web OAuth 2.0 Client ID obtained
         from https://console.developers.google.com/ -->
    <string name="server_client_id">...e4p8.apps.googleusercontent.com</string>

Then inside Activity class:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    ...

    // For sample only: make sure there is a valid server client ID.
    validateServerClientID();

    // [START configure_signin]
    // Configure sign-in to request offline access to the user's ID, basic
    // profile, and Google Drive. The first time you request a code you will
    // be able to exchange it for an access token and refresh token, which
    // you should store. In subsequent calls, the code will only result in
    // an access token. By asking for profile access (through
    // DEFAULT_SIGN_IN) you will also get an ID Token as a result of the
    // code exchange.
    String serverClientId = getString(R.string.server_client_id);
    GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestScopes(new Scope(Scopes.DRIVE_APPFOLDER))
            .requestServerAuthCode(serverClientId)
            .requestEmail()
            .build();
    // [END configure_signin]

    // Build GoogleAPIClient with the Google Sign-In API and the above options.
    mGoogleApiClient = new GoogleApiClient.Builder(this)
            .enableAutoManage(this /* FragmentActivity */, this /* OnConnectionFailedListener */)
            .addApi(Auth.GOOGLE_SIGN_IN_API, gso)
            .build();
}

private void getAuthCode() {
    // Start the retrieval process for a server auth code.  If requested, ask for a refresh
    // token.  Otherwise, only get an access token if a refresh token has been previously
    // retrieved.  Getting a new access token for an existing grant does not require
    // user consent.
    Intent signInIntent = Auth.GoogleSignInApi.getSignInIntent(mGoogleApiClient);
    startActivityForResult(signInIntent, RC_GET_AUTH_CODE);
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == RC_GET_AUTH_CODE) {
        GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
        Log.d(TAG, "onActivityResult:GET_AUTH_CODE:success:" + result.getStatus().isSuccess());

        if (result.isSuccess()) {
            // [START get_auth_code]
            GoogleSignInAccount acct = result.getSignInAccount();
            String authCode = acct.getServerAuthCode();

            // Show signed-in UI.
            mAuthCodeTextView.setText(getString(R.string.auth_code_fmt, authCode));
            updateUI(true);

            // TODO(user): send code to server and exchange for access/refresh/ID tokens.
            // [END get_auth_code]
        } else {
            // Show signed-out UI.
            updateUI(false);
        }
    }
}

You can see the entire code at the following ServerAuthCodeActivity.java

The result, if you use that sample, looks like the following screenshot:

Then, you can follow the steps mentioned at the Google's documentation below (from step #3. Send the auth code to your app's backend using HTTPS POST):

Google Sign-In for Android - Enabling Server-Side Access

UPDATE: from the comments, if you want to get access token directly from android client app, please use the following sample code (replaced with your client_id, client_secret and the auth code)

OkHttpClient client = new OkHttpClient();
    RequestBody requestBody = new FormEncodingBuilder()
            .add("grant_type", "authorization_code")
            .add("client_id", "812741506391-h38jh0j4fv0ce1krdkiq0hfvt6n5amrf.apps.googleusercontent.com")
            .add("client_secret", "{clientSecret}")
            .add("redirect_uri","")
            .add("code", "4/4-GMMhmHCXhWEzkobqIHGG_EnNYYsAkukHspeYUk9E8")
            .build();
    final Request request = new Request.Builder()
            .url("https://www.googleapis.com/oauth2/v4/token")
            .post(requestBody)
            .build();
    client.newCall(request).enqueue(new Callback() {
        @Override
        public void onFailure(final Request request, final IOException e) {
            Log.e(LOG_TAG, e.toString());                
        }

        @Override
        public void onResponse(Response response) throws IOException {
            try {
                JSONObject jsonObject = new JSONObject(response.body().string());
                final String message = jsonObject.toString(5);
                Log.i(LOG_TAG, message);                    
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    });

Please use compile 'com.squareup.okhttp:okhttp:2.6.0' (ver 3-RC1 will have different classes)

With a sucessful response, you will have the following info in logcat:

I/onResponse: {
              "expires_in": 3600,
              "token_type": "Bearer",
              "refresh_token": "1\/xz1eb0XU3....nxoALEVQ",
              "id_token": "eyJhbGciOiJSUzI1NiIsImtpZCI6IjQxMWY1Ym......yWVsUA",
              "access_token": "ya29.bQKKYah-........_tkt980_qAGIo9yeWEG4"
         }