Here are some useful links
add_filter( 'gform_field_value_email_usuario', 'rellenar_email_usuario' );
function rellenar_email_usuario( $value )
{
$current_user = wp_get_current_user();
return $current_user->user_email;
}
add_filter( 'gform_pre_render_3', 'add_readonly_script' );
function add_readonly_script( $form ) {
?>