add_action( 'rest_api_init', 'custom_api_register_with_email_route');

function custom_api_register_with_email_route() {
  register_rest_route('custom-api/v1', '/register/', array(
          'methods' => 'GET',
          'callback' => 'custom_api_register_user_with_email'
  ));
}

function custom_api_register_user_with_email() {

$input_values['input_1_1_3']    = 'mike';
$input_values['input_1_1_6']    = 'geek';
$input_values['input_1_13']  = '12345678';
$input_values['input_1_3']  = 'ram';
$input_values['input_1_4']  = 'mike.geek@example.com';
$input_values['input_1_5']    = '12345678';
$input_values['input_1_5_2']    = '12345678';
$input_values['gform_save'] = true; // support for save and continue

$result = GFAPI::submit_form(1, $input_values );

return $result;
}

{
"is_valid": true,
"page_number": 1,
"source_page_number": 1,
"confirmation_message": "",
"confirmation_type": "message",
"entry_id": "",
"resume_token": "6b8a141b12b8465aad1e428ccb9147b1"
}