Adding c wire to Goodman model GMT070-3B gas

There will be a 24v transformer that supplies power to the board. The thermostat is really just a bunch of switches that controls this 24v supply to toggle on certain relays at certain times.

The transformer has two wires on the low voltage side. One of the wires will become the "R" wire that goes to the thermostat. The other wire from the transformer is the "C" or common wire. The C wire doesn't need to come from the logic board. If your board doesn't have a special terminal to connect a C wire, you just have to splice in to that "other" wire from the thermostat. Also, in many air handlers, the C wire from the thermostat will be bonded to the metal chassis like a ground wire. If that's the case, you can connect your C wire to the screw being used to bond that wire rather than cutting and splicing the wire.

Without much better and more detailed pictures, that is as specific as I can get, but that should hopefully be enough for you to figure it out.