Well, first of all, it says '
STIHL' right on it!
It is a larger, much heavier tool in every aspect. Higher precision casting and finish. More precise, IMO.
The FG-2 is also a stand alone device: it mounts (or clamps) to a work surface, and has a built in vise to clamp and hold the chain while filing, whereas the Granberg (and similar devices), clamp to the guide bar. So the FG-2 can be used in a workshop without having the saw (and gas, oil, etc.) inside. The Granberg can be used on a spare bar mounted in a workshop vise, but is typically used on a chain mounted on a saw.
Shown below with an early version Granberg File-N-Joint (middle), and a current version at the bottom, for comparison.
A more fair comparison would be with the STIHL FG-3, which is also a device that clamps onto the guide bar, but is similar to the FG-2 in size, quality, etc.
Bottom line: they all can sharpen chains. The FG-2 and FG-3 are just really nice versions (and NLA in USA), and so they command a higher price. The Granberg (and similar versions) actually allow more rotation of the top plate angle, while the STIHL versions allow only +/- 0° to 35° (which includes most 'typical' sharpening).
Philbert