Open Discord and go to the channel or server where you want to post the GIF
Click the plus icon or drag and drop the GIF file into the message box
Select the GIF from your device if needed
Add any text you want with the GIF
Make sure the file is in GIF format
Click Send or press Enter
To make a custom GIF, create or edit it in a GIF maker or video editor
Export or save the file as a .gif
Upload the finished GIF to Discord using the steps above