If you'd like to embed a slideshow in Gather for someone to interact with whenever they wish, you first need to upload the slides to a site that will create an embeddable link, such as Google Slides. 

Add Google Slides

Upload your presentation to Google Slides. From the File menu, select Publish to the web. 

From the File menu in Google Slides select Publish to the web.

In the Publish to the web window, select Publish. 

In the Publish to the web window, select Publish.

Confirm that you wish to publish. A URL now displays for you to copy and paste.

Google Slides generates a link for sharing the slides.

From the generated link, remove everything after "pub?" and replace that text with "embed." For example, you’ll get a URL like this, and you need to delete the highlighted text: 

After deleting everything after "pub?", add the word “embed” so it looks like this instead:

Copy the modified link. 

Add the Interactive Object

To embed a video in an object, open the Object Picker from Mapmaker or from the Build pane in your Space. In the Object Picker, select your desired object. Under Object Interactions, select Embedded website. Paste the modified URL into the field. 

Create an interactive object and use Embedded website to paste your Google Slides embed URL.

Save the changes and interact with the object so that you can view the slides. 

A view of embedded Google Slides in Gather.