The guide number (GN) for a strobe light tells us the ability of the strobe to light the subject based on the ISO and angle of view. So, just by comparing GN from one flash to another is not enough. You would also need to compare the ISO and angle of view that they have measured from. Because of this, strobe manufacturers can publish that they have a high GN, but if you do not see the ISO and angle of view at which they measured it, you could be fooled by their marketing scheme.
Take for example...
Nikon SB-80DX - GN 38m (ISO 100, 35mm)
Nissin Di622 - GN 44m (ISO 100, 105mm)
At first look, you would think that the Nissin Di622 is more powerful than the Nikon SB-80DX. But at 105mm, the GN for the Nikon SB-80DX is 56m. Hence, the Nikon SB-80DX is actually a more powerful flash than the Nissin Di622.
The GN for the Nikon SB-80DX speedlight are as follows with reference to the angle of view...
14mm - 17m
17mm - 19m
24mm - 32m
28mm - 34m
35mm - 38m
50mm - 44m
70mm - 50m
85mm - 53m
105mm - 56m
To understand more about GN, you can refer to Wikipedia's Guide Number or Flash Guide Numbers by Dennis Curtin.