Gate camps happen, some can form up in minutes or can just be made up of a roaming fleet heading the opposite direction, even the best Intel channel watcher gets caught by surprise from time to time.
Zoom out and take in the situation. You have 60 seconds to make your decision, so don't panic and flail about for the warp button. You need to assess a few things first.
Your choices boil down to one of the following:
The first option is generally better if the camp consists of very fast, light ships that won't be able to kill you right away but can prevent you from warping. They'll be able to keep up with you if you try to run away from the gatecamp, but will probably be either a couple seconds behind you if you jump back through the gate or or will be stopped by the aggression timer from following you for 60 seconds.
The second option is often the best option when it is possible, since it will only provide your enemy with a fourth of a second or so to lock onto you. However, it can be very risky to attempt in laggy situations, and is generally not guaranteed to work. Be sure to understand this technique completely and have practiced it before attempting it in a real situation.
The third option is for ships such as Covert Ops, Force Recons, Stealth Bombers and Strategic Cruisers1 equipped with a Covert Ops Cloaking Device only.
The fourth option is better if the enemy fleet is made up of a lot of heavy ships that may take a few seconds to lock onto you, giving you some time to escape any bubbles and warp away.
Hold your cloak for approximately 20-25 seconds. This is done to make sure that your session change timer will expire by the time that you hit the gate. If you try to jump through the gate before your session timer expires you will just sit there like an idiot as the Eve client yells at you and the hostiles relieve you of the burden of your ship and/or pod.
This section was stolen shamelessly from the Goons' Wiki.
While waiting for the session timer to expire, right-click the gate and select Add as First Waypoint from the pop-up menu. Approximately 20-25 seconds (just before the session timer and gate cloak expires) after you jumped through the gate enable your ship's autopilot. Immediately click the stargate, press the approach button and mash the hotkey to activate your ship's MWD module. If your ship is equipped with a covops cloak enable this as well. DO NOT activate a non-covops cloaking device as it will significantly reduce your ship's maximum velocity and likely result in you dying horribly just outside of jump range from the gate. As soon as your ship reaches jump range the Eve client should automatically initiate a jump through the stargate, but it never hurts to mash the Jump button when it becomes active. Once you have successfully jumped to the other side of the gate take a quick look around for hostiles and make good your escape.
Protip: Enable the Session Timer indicator in your Eve client settings. It's on the General Settings tab. This will place a tiny whirling clock icon in the upper left corner of the Eve client window which is displayed while a session timer is active. You can also see how much time is remaining on the session timer by mousing over the icon.
This technique works best with ships that are agile and accelerate quickly such as frigates, destroyers and cruisers. Larger ships do not always accelerate to the velocity necessary to warp instantly upon decloaking so there is a chance of getting tackled while the ship accelerates those last few m/s before warping away. In lowsec it renders your ship nearly immune to all gatecamps. In nullsec it will allow you to escape camps which lack an anchored bubble or interdictor with ease and gives you a chance to escape an interdictor camp alive.