So I understand that gates are used for long distance squad movement. But is there a limit on how far the gates can be to move them?

Each gate has to be within range of the squad movements.

Maximum is the 24 ETA range, for your gates and your alliance gates it will be reduced to 12 ETA, however still at the 24 ETA (near the green border when you select your squads).

This way, you have to have a set of gates set up and all you'll have to do is gating them forward.

yup exactly, but it will cost you mana or energy depends on how far the distance between the gate to the other gate or from your colony to the gate..

i suggest you just use it if you need to rush your squads to a certain area..

notice the green circle around the place you will move from? you can move aNYWHERE inside it with the neccessary resources.