In order to share your PowerPoint, you first need to upload it to a site that will create an embeddable link. 

One place you can upload your PowerPoint (.ppt or .pptx file) is to Google Slides. Once you have your presentation open in Google Slides, in the File menu, select Publish to the web. 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, as seen in the screenshot below.

A view of the window that displays in Google Slides after you have published your presentation to the web. A URL displays beneath the checkboxes for auto-advance slides. That is the URL that will be modified and used to embed the slides into Gather.When you have published the Slides, text beneath the title of the window displays "This document is published to the web." Then modify and copy the URL and paste it into the Embedded website field of your interactive object.

You’ll get a URL like this:

Delete everything from the original link that begins with "pub?" and replace it with the word “embed” so it looks like this instead:

Copy the modified link. In your Space, open the Object picker. Identify the interactive object you wish to use, then click Embedded website. Paste the modified URL into the field. Save the changes and test that you can view the slides.