Hello,
32kHz crystals tend to oscillate easily and don't need the high gain.
Above 1MHz the story would differ and I would go for high gain.
Above 4MHz, I would not even wonder and switch the high gain straight away.
I though the datasheet would mention something like this.
Anyway, the user should see the stability of his/her oscillator and act accordingly.
Oscillators are not really part of the microcontroller part, but part of the analog world.
Alban.