Hi,
Yes it is possible, you can set the env variable "group" as <required group name> through the apboot mode and attach antennas so that AP can determine the default gain automatically.
Ex:
If your desired apgroup is "testGroup" then,
apboot>setenv testGroup
apboot>save
apboot>boot
Please ensure that APs fixed with antenna otherwise you have set the gain value manually(setenv a_ant_gain <gain>) and which is very difficult because the gain value will be in coded format :)
Please feel free if further clarity required