It does seem that Unite are only balloting LGW crew who took part in strike action:
http://uniteba.com/ESW/Files/MESSAGE...W_STRIKERS.doc
MESSAGE FOR ALL LGW STRIKERS
If you are a Gatwick crew member from either the BASSA or Amicus sections of Unite, and you took part in the recent industrial action, losing your staff travel, but, you have NOT YET claimed strike pay or have only claimed within the last 7 days, please email: ** ASAP.
This is to ensure you are included on the list of members to be balloted over the coming weeks