Hi,
I came across videos online showing how to use two "secret" channels (marked as F1 and F2 in NAZA controller) to control gimbal.  Are there more spare/unused channels that I can use, for example to take a snapshot with a camera?
I can think of 4 channels to control movements:  1) altitude (up/down),  2) X-direction (forwrd/backward),  3) Y-direction (left/right),  4) rotation(cw/ccw)
What other channels are for (including spare ones)?
How many channels are built in into the stock radio controller and how many of them are used?
If I use F1 and F2 to control gimbal and there are no spare channels left, what are my options to take snapshots with a camera and operate zoom on the camera?
Thank you.               

There are 7 channels on the stock controller. 2 for each stick, 1 each for the switches and 1 for the gimbal pitch lever. There's no way to get any more channels out of it beyond the old modification to get the gimbal pitch lever working on older Phantom transmitters.
If you upgrade to a different transmitter and receiver combo then you will get what you want.
I run an FrSky Taranis on my Phantom 2 and S900 with the X8R receiver. This gets me 16 channels although I'm only using 8 of them with the Phantom.

ri.rilot, thank you for your response.
I now understand that 6 channels that are used are: 4 for motion (altitude, x-direction, y-direction, spin) and 2 for switches.
I see people saying that it is possible to connect spare F1 and F2 outputs from Naza controller to gimbal pitch and gimbal roll.
That makes it 8 channels total, not 7.
Does this mean that there are actually 8 channels in the Phantom receiver and Naza, but the stock DJI controller only supports 7?
Thank you.

We use a DT7&DR16 RC system. It means the transmitter support 7 channels and the receiver support 16 channels.

Why would you want a receiver that supports more channels than a transmitter does?