Many apps and sites that take video do not play GIF files. You pick the GIF, and it posts as a still picture, or it is refused. The usual fix is to turn the GIF into a short video, which every video uploader accepts, and which is also a much smaller file.
This converter reads the frames of your GIF with their timing and encodes them as an H.264 MP4, in your browser. A GIF is often only a second or two long, so you can repeat it until the video reaches four seconds or more. You can also double the size. The GIF is never uploaded and there is no watermark.
Why post a GIF as a video
GIF is an image format from 1987 that stores each frame as a full picture with at most 256 colors. A video stores only what changes between frames, so the same clip is usually several times smaller as an MP4, with better color.
Video uploaders also expect a video file. If a GIF does not animate when you post it somewhere, converting it is the fix. We could not confirm on Instagram's own help pages how it treats GIF uploads today, so test with your account.
Length and looping
A GIF loops forever; a video ends. Repeating the GIF a few times gives viewers time to see it. Pinterest asks for video pins of at least 4 seconds, which is why that is the default here.
The video has no sound, because GIFs have none. To add a title, use add text to video. To reshape it for a tall feed, use the Pinterest video resizer. To go the other way, use the video to GIF converter.