To embed a Miro board into Gather, you need the embeddable link for the board (which is different than the board's regular shareable link). In Miro, select Export, then select Embed.
Copy the URL from the embed code between the quotation marks after src=.
Once you have the link, paste it into the Website field of an embedded website object!
Note: For more detailed instructions on how to obtain your embeddable link, please visit Miro's article on Embedding Editable Boards into Websites.