I do not believe Lemmy internally handles things like gif or videos or photo album posts currently. To do that is best to just use a third party service and post the link to it. I'm sure other lemmings can point you to good services for this.

If you put a direct link to a gif or video as the "link" of your post, I think it embeds automatically. But Lemmy right now does not support uploading videos directly.

deleting this post per asklemmy rule 3 (use !lemmy_support@lemmy.ml for support questions)

Albums aren't supported yet though, you'll have to post extra images in the post body for now

One of the "editing" icons under the post/comment window will allow you to upload images directly to Lemmy. I suspect it would accept a GIF, but I haven't tried that. It does not currently support videos directly, but they can be done by linking to another source, as was already said.

Lemmy is built by volunteers and people are paying for servers out of pocket. Storage costs money so it's unlikely you'd ever see local hosting of large files like videos. Some instances will locally host files under a certain size limit, but it depends on the instance. There's plenty of third party file hosts so you can just upload your file to one of those and link to it.

Those are all possible, the options on how to do each aren't clear or very versatile, yet.