How do you judge gaps in traffic?

First, you need to know how fast the oncoming traffic is going. Obviously if traffic is moving slowly, you don't need a large gap. If traffic is moving fast, you need a larger gap.

Secondly, you should know how long it will take you to speed up to the speed limit.

Finally, if you are not sure if you can make it in a gap, don't try to. There will be another gap. The people who are behind you will wait. Be safe, be smart.