To crop an image file...

• Open the image in GIMP

• Click on the crop tool (interlocking 90 degree angles)

• Drag a square around the part of the image you want to keep

• Make use of of the Fixed Aspect ratio or other options from the left-hand toolbar as necessary

• Make use of View  "Show Guides" if necessary

• Double-click the area inside you rectangle

• Click File then Save... / Save As... / Overwite / Export As... as necessary