Making a seamless gif in a nutshell:
-Film: At least 1080p resolution.
-Edit: Try and keep the length at a few seconds or under 10 as a best practice, keep the file size under 1MB to avoid uploading to social site issues, ensure lighting is the same in the beginning and ending frames so the GIF doesn't appear to loop or restart.
-Use a GIF generator: Giphy is a popular choice (https://giphy.com/create/gifmaker), add text, or download/share.
Check out this article, it has more in depth info https://idearocketanimation.com/14835-animated-gifs/
Here is an example of a seamless GIF from Giphy