The "thunk" is not a symptom, it is what you want to hear (and more, really, to feel) when you advance the film. If you don't hear or feel the Yashica thunk (and it is unmistakable), then the camera likely needs service. I am guessing that your original G probably needs a POD replacement, while the later GS is likely OK.

The battery check is on a different circuit than the meter and over/under lights, and, unlike them, it is susceptible to variances in voltage. It is common for the battery indicator to fail when using alkaline batteries after they dip below their original 6 volt voltage, as it was designed to work with mercury batteries, which have a more stable voltage during their usable life.