If you want user to be redirected back to your page from payment gate then you need to init flow with 3 parameters:
If returnUrl wasn't defined then user will end up reservation process on our Thank you page.
<body>
<div id="main"></div>
<script src="https://yourcompany-perfectgym-address/ClientPortal2/sdk"></script>
<script>
PerfectGym.ECommerce.init({
flowId: 'b64d45ff3',
element: '#main',
returnUrl: 'https://github.com/PerfectGym/ClientPortal.ECommerce'
});
</script>
</body>