On macOS Sierra, the most recent version of Gapplin, 1.3.1 (275), fails to open windows. Dragging an SVG to the icon fails and opening a SVG file using File - Open also fails, rendering the entire application unusable. At this time it is not possible to open any files using the program.
Quitting and restarting the application does not fix the problem. Rebooting the computer does not fix the problem.