I believe that there is a 6 tick period where you can't use it after you hve upgraded an outpost to a gate.
However, to use a gate, the starting and ending spot both must have gates (either a gate outpost, your colony, or an allied colony with a gate built). First, click your squad(s) and click on your destination. Below the normal "OK" button to send your squads should be another option that says "Jump to Gate". Click this button and your squads will instantly move to the destination.
The cost of using a gate is half of the ETA ticks in energy per squad.
Example: You want to send 3 squads to a gate outpost 8 ticks away.
8/2=4, 4*3=12
The cost is 12 energy.
Hope this helps
