MPG does not play in browsers or on phones and is far larger than MP4. GIF plays and loops automatically in every browser, chat app and email client. MPG is at home in DVDs, Video CDs and old camcorder footage; GIF is the usual choice for short looping clips and reactions. GIF has no sound, so the audio track is dropped.
The defaults suit most files. Under Options you can set the frame rate, set the width and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because MPG is not handled by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB. A GIF covers at most 30 seconds, counted from the start point you set under Options (the beginning of the video by default), so a longer clip is cut there. The defaults are 12 frames per second and 480 pixels wide, which keeps the file a sensible size.