“antenna gain” is a correction factor for the TXpower (relative setting) . It has nothing to do with the physical antenna gain, except it’s name and default (minimum) value.
MT removed the possibility for relative power setting. Do the math yourselves after collecting country and device limitations. (Or after checking the used TXpower when set to default.)
Did you set the “antenna gain” as relative parameter before the upgrade? It will be invisible now. Correct it with CLI.
http://forum.mikrotik.com/t/antenna-gain-missing-in-6-46-8/145162/1