There are 3 ways to get gear bags afaik
1. PvP Dailies
2. PvP Weeklies
3. 200/200 Merc/Warzone Commendations.
In the bags you either get 3 Centurion commendations or 1 commendation and a random token to exchange for that appropriate gear slot. Its very RNG based. I, myself, have picked up around 10 tokens (including weapon, chest, helm, and legs) while I know people who still haven't seen their first token.