Here are the steps for cropping an animated GIF using Gimp.
Open the GIF file in Gimp using File ➾ Open
Press Shift+C to select the crop tool. It can also be selected via Tools ➾ Transform Tools ➾ Crop or by clicking on the crop icon on the Tools Pane.
Select the portion of the image you want to keep by clicking and dragging.
Press Enter to automatically crop and resize the image to the size of your selection.
Choose File ➾ Export As to save the cropped image.
For saving animated GIF, make sure to select the As animation checkbox.