So I explicitly asked them what the antenna gain setting should be configured to when attaching a 2.1 dBi nominal gain antenna and a MikroTik 868 6.5 dBi gain Omni antenna in order to maximize transmission power while staying in the legal limits (when using a short/the included short cable). The answer was to set it to 2 and to 6, respectively.
They also clarified that the unit of this setting should read dBi (as documented in the online manual) and they want to fix the UI label with some future system update.