Will it be possible to ad GIF without requiring code ?
PS : Thanks again Internet Janitor for this amazing tool btw.
Have you seen the GIF contraption in the Contraption Bazaar thread?
If you haven't used contraptions before, this subthread might be helpful.
As written, the GIF player contraption I provided in the above link is designed to resize itself to match the size of an imported image, rather than resize the image to match itself. If you're feeling brave, you could always edit the contraption prototype to change its behavior.
Based on your original request, I'd recommend scaling your GIF images down to an appropriate size using an external image editing tool (imagemagick, photoshop, GIMP, etc.) before importing them into the GIF player contraption. For reference, Decker's default card size is 512 pixels wide and 342 pixels tall.
Do keep in mind that large GIFs can increase the size of a deck considerably!