Nick here from Shopify.
There are a few different GIF makers you can use to make your GIF's. These aren't related to Shopify, but the ones I would lean towards can be found below:
With regards to how to add a gif to your product page for the Debut theme, I spoke to our theme support team about it. They mentioned that you should be able to add a gif to your product description section by clicking on the ‘Add image’ button in the RTE settings. You can see how this looks via the screenshot below:
All the best, Nick