You can either long press the heart to add custom reacts or start typing a colon : like the @ for user lookup to look up custom reacts by name. And then press the icon on the right to paste in the url.

You have to be on the latest amethyst version.

If you have gifs installed, then when you type ":", it will show a list of gifs to pick from and then you can tap the "enlarge" icon on the right to insert the full size gif into your post.

Press : and then select the gif with the button on the right in the list that pops up