Warp gates vastly decrease the traveling time between two stars, as long as both stars have a gate present. The time to travel between two systems with a warp gate is approximately a third of the normal flight time, ie: a 12 hour journey would take 3-5 hours with a warp gate either end.
The warp gate's cost is inversely proportional to the planets resources, meaning a higher terraforming level reduces the cost of the warp gate. The base cost on a homeworld (with 55 resources) is 181 credits. On a planet with 10 resources, the cost is 1250 credits.