Do you hear the fuel pump start up?. Does it sound normal or sound like it is in slow mo. If in slo mo, then that would indicate the AMPs issue, as it draws big amps compared to lights and gauges.
It is pretty normal to see a battery with a surface charge of good volts, then on a draw down test it goes from say 12.7v to 3v and reovers to only 5v, not enough to even bump the starter.
If you have a battery charger with jump start capability you could try that. If she cranks, then it's the dead cell issue. You will need to replace the battery. If you still get nothing I would say the battery is not the problem.
Your local Autozone can test the battery but, I would take it out of the car and bring it in. They also sell an Optima that should fit, but it does not have the extra lip of the OEM battery on the base. When looking for my car I saw 2 used GTs that had regular Optimas mounted in them.