This feature is accessible from the More Effects dialog.
This allows you to invert one or more RGB channels in the image or selection. Select All to change a photo into a negative and vice versa. This generally looks quite funny.
To quickly invert all channels, you do not need to use this dialog. You can instead access the item in the or menu. The item is only displayed in the menubar if you use one of the selection tools. Additionally you can reach this action from the context menu opened with a mouse button click in the image area.