On a computer, open your site in Google Sites and go to the page where you want the widget.
On the right, click Insert, then click Embed.
Click the Embed code tab and paste your code. Click Next.
Check the preview and click Insert.
Drag the blue handles of the embed box until the whole widget shows without a scroll bar.
Click Publish.
Google Sites shows embedded code inside its own frame. If your widget uses Lightbox as Photo Link, the lightbox can only open inside the embed box. Make the box large enough, or set Photo Link to Photo Popup on the Advanced tab of your Pro widget.