A filter won't turn it back to a normal camera, unless you can get one that exactly matches what was in the camera.
On the Panasonics I've done, there are two filters that need removing, and one of them is replaced with clear glass.
You also loose the ultrasonic dust shaker, so it is probably best to get a G3 or G5 and convert it. The G3 is ideal, as it has just about all the features of the G5 you would use, and it is small, so convenient to carry as a second camera.
They used to be available for around £130 converted, which is quite a good price as the camera has already been sourced and checked, and the glass to replace the one filter would cost you upwards of £30 to buy one, I was quoted £130 for two pieces by a big optical firm (I laughed and deleted it), not easy to find.