Step 1: Upload your video/GIF to the CONTENT section in your Shopify menu. It's on the left hand side under discounts.
Step 2: In the menu page, navigate to the far right of the file you want and click the link icon to copy the file link.
Step 3: head to Unicorn and setup the Free Gift/Buy X Get Y for your GWP.
Step 4: Paste the file link in the 'Product Media URL' bar
And voila, that's how easy it is to add a custom GIF/Video to your cart - without needing to edit the product itself!