How exactly do the lower and higher bracket BG-s segregate?

I recently got upped into the higher bracket PVP with 5.1k GS. A lot of my gear isnt even PVP oriented, and I get absolutely ****ing mauled by every enemy player I come across now. I dont understand what the quota for me upgrading to the high bracket was, is it just based on gearscore or is something else involved here? I feel completely useless in every battleground I join now, and its that much harder to get gear now when all I face are Blackrock players with full PVP sets. Is it public information how this segregation is made? Should I not get gear from heroics next time and only farm BG-s so I can be better prepared for when I get dragged into the full pvp set pool?