//automatically log user in after activation
function auto_login_user( $user_id, $user_config, $entry, $password ) {
    $user = get_userdata( $user_id );
    $user_login = $user->user_login;
    $user_password = $password;

    wp_signon( array(
    'user_login' => $user_login,
    'user_password' => $user_password,
    'remember' => false
    ) );
}
add_action('gform_activate_user', 'auto_login_user', 10, 4);