That comes down to color temperature of the ambient light. Flash always has something similar to daylight (5500-6500K), so you need to use conversion gels from daylight.

Most useful gel is CTO (color temperature orange), which will color daylight to tungsten (3200K). Usage is as follows:

1. Stick CTO gel on flash
2. Set color temperature to tungsten
3. Shoot

This has two possible effects:

• If ambient light is tungsten, everything will look just normal
• If the ambient is normal daylight, you foreground will have proper color, and everything else will be toned to blue. This can provide nice color separation effect (example)

Other usual gel is window green, that converts daylight (e.g. flash) to fluorescent-like green. Usage is similar to full CTO.

People also use half- and quater CTO, which convert daylight to 3800K and 4600K. These can be used for less-visible separation, or to warm up light for portraits. (Usual scenario: light some stuff with ungelled flash and the person with 1/4 or 1/2 CTO-gelled flash; an example with a bit more complicated setup can be seen here.)

Full description, examples and links to much more can be found in article at Strobist, overview of various gels can be seen at Rosco.