What value should be this parameter? always zero?
Or should be the value of each antenna (for example 16 for SXT, 7 for omnitik, etc.)

Thanks!

Put there real value of antenna gain. If you need to reduce the to power, use bigger value. This value has effect with regulatory domain and country set.

I don't fully understand this...

Does changing this setting actually have an effect? OR is it just like setting your country setting on the wireless interface (i.e. no effect at all).

It has effect. Also setting the country has effect as there are different channels allowed and different tx limits in different countries. Antenna gain just says how much power should be deducted in calculation formula when setting the tx power automatically according to country regulations.

If you want to follow your country law, set country and antenna gain to actual values. "wireless mode" has to be "regulatory domain" for these settings to have effect.