You can add gifs either by attaching a GIF file using the same method you attach images, or by using a GIF picker on some phone keyboards, or by pasting the link to a GIF on Giphy or similar services (GIF links are embedded automatically.)
You can change servers by doing these steps, but it's only worth doing them if you have followers you want to carry with you to the new account: https://fedi.tips/transferring-your-mastodon-account-to-another-server/
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