Additional gateways need to be included into Store via an extension so you would need to create an ExpressionEngine Extension. Store has a sample extension of what the gateway should like here with the proper folder structure and files needed to register an additional gateway.
You will need rename the ext.store_check.php file to match the namespace of the sisow gateway. Once you have done that you will need to add the folder to your third_party folder and enable the extension in the Add-ons > Extensions age of EE and configure it however needed before you can use it on the site.