FFmpeg is a video and audio converter that can also grab from a live audio/video source. It can also resize video on the fly without compromising the quality.
Make sure to install ffmpeg using your distribution’s package manager:
sudo apt install ffmpeg
Once you have selected the video that you want to convert, open your terminal and change directory where your video is saved. Below is a general principle, where input is the actual name of the video, following by the video format and the name that you want your gif to be.
The output name can be something totally different to the input name, but I tend to use something similar, as it helps to identify it when you have a folder full of files.
ffmpeg -i vid.mkv -map 0 -c copy -c:a aac vid.mp4