A deep gnome has darkvision out to 120 feet and low-light vision. What benefit is there from the low-light vision? The only thing I could think of was with a bulls-eye lantern or daylight spell, he would have some shadowy illumination out to 240'. Otherwise, it seems kind of pointless.
Low-light vision lets you see color!
Within the 120' radius of a deep gnome's darkvision, he only sees in black and white without the presence of a light source. Within the radius of a light source, folks can see in color whether it's inside their darkvision range (if any) or not. Low-light vision doubles the radius of illumination from all light sources, thus doubling the range the deep gnome can see color around a light source. This is useful.
Low-light vision has great advantages in distance!
Low-light vision's range is not fixed, nor is it centered on the gnome: it simply doubles the radius of a light source, no matter where the light source is or what its radius. This lets the gnome see things which would be hidden to creatures with only darkvision. For example:
If a torch sconce (20' radius of bright illumination, but 40' for the gnome) is 200' away from the party, then a thief sitting 30' away from the torch can make Hide checks to avoid detection by both party members with normal vision and ones with 120' darkvision. However, that Hide check automatically fails against the gnome, for whom the thief does not have the benefit of shadowy illumination to hide in.