[Metalstorm]

If you know where the gate loop (the wires beneath the asphalt) are, there are two things you can try.
1.
Place the kick stand down directly on the corner of the loop where it is the most sensitive. I would suggest this for traffic lights as well but the engine shuts off as soon as the stand is lowered unless it's in neutral.

2. (this works better)
If you have something small, heavy and metal, you can place it over the loop to trigger the gate then kick it out of the way on the way out.

At work I keep half of a broken patio umbrella stand in the truck for just this purpose when I'm sealing driveways that have electric gates.

On Edit: I like spdklls's solution for the traffic lights. I've always wanted to try one of those signal sorcerer thingies, basically just a magnet mounted under the bike. This would probably trigger the gate as well.

 

[Metalstorm]

If the gate requires a remote to open you're S.O.L. unless you get yourself a remote. At least for entering. Most gates will open to let people exit.

If the gate is set to auto then it opens automatically when the loop's (the square cut is where the wires lay) magnetic field senses a vehicle. The corners are the most sensitive.

There are a couple companies that market magnets for mounting onto bikes.
I'm currently thinking about this one. It's small and requires roughly 1 square inch of room for mounting.
Red Light Changer

No guarantee that it'll trigger the gate but it won't hurt and may help at traffic lights.

Edit: Is there a keypad box near the gate? If so you simply need to get the code from whoever is in charge of the complex.