Allow Site Embedding (in iframes) - classic Google Sites
Do you want to embed classic Google Site on another site (in an iframe)? Classic Google Sites provides a setting to allow site embedding so you can allow your classic Google Site to appear in another site.
Use the More button (it looks like a gear/cog), then Manage Site (or keyboard shortcut g then m)
Scroll to the bottom of the General settings to find the Security section.
To allow embedding of your site enable the
Allow embedding of your sites in other sites option
Use the red Save button at the top of Manage site
That's it, you're done!