How Stargates Work
Stargates can instantaneously transport all of your ships in their system to any other system that you own. They are expensive — only the engineer costs as much to build and maintain. If you know how to use them, though, the benefits are well worth the cost. Stargates are my personal favorites, and it is a rare game indeed where I do not choose them with one of my first three tech selections. Their proper use makes for not only a devestatingly effective offensive attack, but a paralyzing and demoralizing psychological attack as well.
Stargates can send ships to any of your planets, regardless of distance and regardless of the gate's BR. Almost all players, then, build only BR-1 stargates. Building them at higher BR offers no real advantage (unless that system is attacked) and is prohibitively expensive. There have been efforts to link stargate performance to BR, but these implementations are only in the non-version 2.8 game systems.
Note that stargates will only transport your ships, not your enemys' and not your allies'. Also important to remember is that transport takes place after movement and battle, so you cannot gate ships and have them fight at their destination. But, ships can fight in the stargate's system, and if the gate survives it will transport them to the target planet after the battle is over. Also, satellites and mine fields are not transported, they will remain stationary over the planet where they were built.
Be sure to coordinate the timing of your ship builds to your needs. A stargate will transport all of your ships in that system unless they are set to either move out of the system or to terraform the system. This means that you cannot build a large fleet and plan to transport part of it and leave part of it for defense. This is balanced, however, in that you don't have to wait an exta turn after building ships to transport them, they will be sent the same turn they are built.
One clarification on the order of what happens in a turn. The FAQ states that gating and the engineer's function of opening/closing gates takes place last. But what happens when you try to combine these two? This might be a rare occurrence, but most people don't know because they have never been willing to take the chance on expending that many resources in a game and wind up wrong. The answer is that the engineer will open/close the jump, and then enter the stargate. You could therefore close the gate on an attacking fleet that's poised over your Home World and transport your fleet to another front. Now the enemy must dismantle his fleet next to your Home World and rebuild another one, or face the prospect of your fleet marching up to his Home World nuking everying in site.
As a result of the build/move/fight/gate sequence, stargates will actually transport ships as they are being built. This allows you to overbuild a fleet and gate it to its destination, knowing that it cannot be destroyed as it arrives. It can then be sent into the enemy's planet the next turn at full strength. This can actually result in such odd behavior as having your fleet appear in the same planet as the enemy's with no combat taking place.
This same ability allows you to overcome a defeat in battle and quickly recover, even if the battle was over your own planet. The attacker will likely nuke the planet the next turn, but you can still gate in ships to that planet as he nukes it. The planet will be nuked, but your ships will arrive — imagine the enemy's surprise and disappointment at not being able to follow up on his victory. If you include a colony ship in the fleet you can quickly recolonize and get your offensive back on track. The enemy fleet will often be low strength after the battle, and perhaps even disbanded except a lone nuking ship. This makes it easy to destroy what remains and recolonize.
This ability is particularly useful when you have a planet on the border of unexplored space. You can gate a fleet containing a science ship to the border, and the scout will be able to explore the jump the next turn. Note the importance of this feature — it *guarantees* that you will be able to explore the jump because the science ship will arrive after combat is resolved and so cannot be destroyed by an attack on your planet.
Stargates are the best way to coordinate an aggressive attack. They allow you to flood the front with ships without waiting for them to move up from builders in the rear. Even if your initial fleet is destroyed you can often gate in a second fleet that can nuke your opponent's forward planets before he can rebuild and move his fleet to the front. You can also gate in a backup fleet right behind your main fleet to move in and support the nuke.
The Rolling Staragte Attack
The key to a rolling attack is to keep moving your border forward, which means making sure that you include either colony ships or troopships with your fleet. You can then occupy the planet you just nuked and use it as your next stargate jump point. You can therefore keep your fleet at full strength by constantly gating reinforcements to the front, unlike the traditional attack where replacements must be moved up to the front from builders several planets to the rear.
More Nasty Surprises
Ships being gated need not be in the stargate's system at the beginning of the turn. If you move ships in adjacent planets into the system with the stargate, they will be transported along with everything else. I have seen this throw even experienced veterans into complete confusion, especially if they can see the fleet but not the stargate. It will appear as if you pulled your fleet back into hiding and have a second fleet somewhere else. This tactic is particularly effective with single or small groups of cloakers.
Using the same feature, stargates can actually be defensive in nature! Imagine an attacker's fleet in the planet 2 systems from your Home World and your fleet in the planet between them. You know you can't stand up to his fleet this turn, but you want to stop the science ship he has with him from exploring too far. You can retreat your fleet back to your Home World and have the gate set to send the fleet back to that same planet (probably with a few more ships that you build in order to match his fleet.) All of these ships then arrive back in the same planet they came from at an overbuilt maintenance level, but none are destroyed! This then lets you shadow his exploring science ship the next turn and defend your planet against the bulk of his fleet. Your opponent will likely be confused about how all of his and your ships are in the same planet unharmed, and he may waste time trying to figure that out rather than reacting to your ships.
Stating the Obvious
Because your stargate will most likely only be BR-1, you have to protect it. This is most commonly done by locating it in a planet which your opponent cannot see. Even a BR-1 cloaker can spell disaster if it uncloaks when your gate is unprotected. You may even wish to engineer off your gate planet to insure its protection. One final word of warning, if you do build your gate at a planet other than your Home World, be careful to build your ships in that same planet. Since the build screen defaults to building in your Home World you have to remember to change it each time.