G to C trick not working for cooling. Honeywell RTH9080WF
Your original wiring looked like this, where the top circles represent the thermostat terminals and the bottom the furnace.



To properly pull off the "G wire trick", you have to locate the transformer in the furnace. You then have to determine which leg feeds R, and which does not.  The leg that does not feed R, will be where you connect your new C wire.



Once you have all that hooked up, then you'll install a jumper between the Y and G terminals in the furnace.



Once all that is completed, you'll have to make sure the motor is spinning at the correct speed when in cooling mode. If it's not, you'll end up with inefficient cooling. Which can cause poor performance, and/or damage to the system.