The feature is gradually being rolled out, as is often the case, so it’s not available for all users just yet. If you do have access to it, you’ll find it to the right after opening the standard camera feature.
GIFs recorded are only a few seconds in length, but all the filters, effects, and frames from the standard camera tool can be applied. Once you make a GIF, it can be posted your Facebook page or added to a story.
Facebook hasn’t said when it plans to make the feature available for everyone, but it seems it’s only being rolled out to iOS users for now.