Steegle.One Intranet for Google Workspace

Allow Site Embedding (in iframes) - classic Google Sites

For new Google Sites there's no option to allow new Google Sites appear in iframes - see Compare classic to new Sites.

Instructions

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