You can find complete documentation for SMS Gateway from here. If you are using cPanel, then you can follow this tutorial to install it on your server.
If you don't have web hosting and domain to host the app, then you can also use it by installing it on your PC. You can do this by following tutorial from here.
If you want to enable PayPal and other SAAS features then make sure you follow instructions from here after the installation.
It is required to create a separate cron job for renewing subscriptions. It is required if you want to create subscriptions manually or use PayPal. If you followed all steps from above then you don't have to do this, but if you are only want to use manual subscriptions then you can set it up as shown here.
You can find frequently asked questions from here.
If FAQs doesn't resolve the issue you are experiencing then you can open a support ticket from here.