Hi,
Firstly you have to upload the gif in the shopify admin in the bottom left by clicking settings
it will bring you to a new page and you’ll want to click on “Files”. Once you upload, it’ll provide you with a link which you can add as the src attribute in an tag.
you can copy the link & past on the page template wherever you want it, may be I think it is custom html or any
Given that you tried to upload it already, I assume you’re using something that comes with your theme in the customize section.
In which case they may not allow gif uploads, so you would have to do it manually and then insert it with some HTML