Holding down the left mouse button on an image allows you to scroll the image.

The mouse wheel will scroll the image up and down.

Clicking the middle mouse button will toggle the auto zoom on/off.

Hold down the Control key, then either use the mouse wheel to zoom in and out or left click to zoom in and right click to zoom out.

The mouse wheel, used while holding down the Alt key, will scroll the image horizontally.

When in Browse mode, clicking an image switches into View mode and shows that image.

When in Browse mode, scrolling the mouse wheel will scroll up or down the thumbail view area.

If the Mouse Behaviour option in Settings->Configure Gwenview is set to Browse, scrolling the mouse wheel while in View Mode will move you through the images in the folder.