Some time ago, Google introduced some changes in their Google Sites setup. This includes a change to what we call 'framed forwarding' and what Google calls 'embedding'. By default, this option is now disabled on Google's end.
You will need to go into your settings and enable embedding for your Google Site on your Freenom domain. You can find the embedding setting in Manage Sites > General > Security on sites.google.com.
If you do not change this setting, your Freenom domain will continue to show a white/empty page.