That’s probably the case. Try Camera2 API Probe and see for yourself. If it didn’t support Camera2 API, then GCam wouldn’t open at all.
I don’t know which ports you have tried. I use BSG and when I go to Camera settings → Photo → Camera photo resolution and select the Max resolution option, I get the full 12 MP quality.
If you mean the 48 MP quality, then you should know that even in the stock camera app it’s a bit of a hack (it probably requires some special capture mode). And it will probably never be accessible through the API.