When it comes to Iris/gain value in video cameras, what does 0dB mean? Is there gain in 0dB? Is the video signal being amplified?

Measurements and ratings given in decibels can be confusing 

because they require a reference, 

and in some cases that reference is Implied rather than Given. 

However, EXCEPT when zero dB itself is used as the reference, 

it Always means that there is neither gain nor loss --- 

the output level is the same as the input level. 

When zero dB is the reference, other factors must be stated. 

I know ... it makes one shake one's head in bewilderment. 

0 dB is the manufacturer's design electrical gain reference value for use with outdoor sunlit subjects.  however evening or indoor subjects may be too dark, even with the iris fully open, so you can compensate by setting the gain to a higher value, ie +6 dB.  this is the exact equivalent of increasing the ISO setting of a still image camera, and causes the same increase in visual noise.  some video cameras also have an option of decreasing electrical gain, ie -3 dB, for better quality when there is an abundance of illumination.

there will be little gain on 0DB...howerer u can set gain on minus3db on most camcorders...so -3db is no gain db and gives u cleanest image

"Or is there no gain at all in 0dB? "

That's correct - it's the equivalent of the exposure compensation in a stills camera.