The zoom tool in GIMP magnifies a selected part of an image.
Steps to use Zoom tool are:
• You can get to the Zoom Tool from the image-menu through: Tools > Zoom, or by clicking the tool icon 
in Toolbox.
• Click on the image to increase the size of the image.
• To reduce the size of the image, press the CTRL key and click on the image. The mouse pointer will change into a (-) sign along with the magnifying glass.
