Holding down the left mouse button and moving lets you drag the map in any direction, providing a quick way to move around.

Clicking with the right mouse button will center the current mouse position.

If you have a mouse with a center wheel, you can roll the wheel to zoom in and out (changing map series), which is extremely handy.

Clicking with the center mouse button is bogus. At present it prints the current location (in the terminal window used to start gtopo, if it was started that way), but future behavior is not guaranteed.

That is the default behavior. You can use a settings file entry to make the left button do the centering also (in which case you loose the much more useful dragging capability). You can also use a settings file entry to make the right mouse do the zooming, in which case just clicking with the right mouse will zoom in, and clicking while holding down the control key will zoom out.