Before you create a video with GOS camera you can lower the resolution for smaller file size. After you've shot a video you have a lot more options to compress/transcode/convert resolution if you're willing to transfer the video file to desktop/laptop - e.g. ffmpeg, Handbrake, VLC...
The only FOSS Android video compressor app I'm aware of is FFShare - https://github.com/caydey/ffshare - but don't expect meaningful file size reduction compressing already compressed video - No harm in testing it though.
Otherwise, if you're determined to create and convert video solely on your GOS device you can search Google Play for "Video converter" or similar terms.
Please keep in mind that iirc MMS (if you're using GOS Messaging) file attachment limit is somewhere between 1-4 MB which is very, very small for video - so the video would have to be extremely low resolution and extremely short.
Maybe consider other file sharing options e.g. Signal, https://send.vis.ee/, https://wormhole.app, Onionshare, Bitwarden Send, or whatever you're comfortable with.
I hope this answers your question and was helpful.