We knew from before that:
a) You can Travel short distances without knowing a place (The first mention I can find is in aCoS, when Rand attacks Sammael)

b) When you Travel to a place, you learn it, so making a Gateway from that place is possible.

So, assume you're in a place you don't know. Typically, people would skim away from that place.

However, Rand uses points a and b in conjunction. He hops a short distance through a Gateway, and since he has Travelled to this new place, he knows it enough to make a Gateway from there to anywhere.

There are, however, obvious reasons why this cannot always be used. If you want a quick escape from a place, Gating into a nearby area, and then making a new one from there will take too long, and you'll still be in danger till you can get out of the second gateway.

Also, in a city, there's no way you can use this. Who knows what you'll cut into while making your first gateway?