This code can be used as a base to create your own simple custom payment gateway for WooCommerce.
If not used in a custom plugin, you need to add this code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Please don’t add custom code directly to your parent theme’s functions.php file as this will be wiped entirely when you update the theme.
https://gist.github.com/woogists/c7675d09d3bd79893fbbba5e93b284c9