You can find the pix2sig help file in: ~ ▸ Documents ▸ Pd ▸ externals ▸ Gem
You might be getting "no method for gem-state" if you connect an object that takes signal input to the first outlet of the pix2sig object - the signal outputs are outlets 2, 3 and 4. Outlet 1 is for the gem data to pass through.
"bad vector size" could be because you've used block~ or switch~ to something other than the vector size you've set in your prefs. I believe PD adc and dac need to run at the same vector size as what's in the preferences. To do this you could route the signal into another pd window, and there have your dac object.