Record a clip of what you want to make a gif of, then go to:
https://ezgif.com/video-to-gif and upload the video. Pick the start time and end time for the gif and then pick the best framerate for that duration, if you want a 25fps gif you aim for a time of 10 seconds or less (as in start and end time being no more than 10 seconds apart), then click convert to gif. You will now have a gif thats far too large for CB, click resize under your new gif, in height put 80, it would be super rare to need to set the width but if for some reason you have something ridiculously wide you can set the width instead to 250 and leave height blank. Now click resize image. As long as the image is 250x80 or less and less than 1MB in size then it will work for CB chat.
You can use "optimize" to reduce the file size some if you are over 1MB in size.