Open GIMP
Go to "File" "Open" and select your GIF.
Select the "Crop Tool" from the toolbox.
Adjust the crop area to your desired size.
Click the "Crop" button.
Go to "File" "Export As" and choose GIF as the format. Save your cropped GIF.
FFmpeg
Use the following command to crop the GIF, replacing input.gif with your GIF's filename and specifying the crop dimensions:
ffmpeg -i input.gif -vf "crop=w:h:x:y" output.gif