Regarding doing the General Paper rather than A level General Studies as many others did, we had to make a choice and the A level seemed too much like hard work on top of my other studies.
I did my A-levels in the mid-70s. The school put on General Studies "lessons" for an hour a week. We were often sent to work quietly in the library instead! After a few months of this, it became clear to me that the General Studies paper was just that, and any student with a broad general knowledge should manage a decent pass, so I stopped going to the lessons and went to the library anyway. I did no work to prepare for the exam and got a good grade. As Alison says, if you were lucky and got a question or two on pet subjects, it was a free A-level.
We had 3 or 4 40 minute lessons classed as General Studies on various subjects, which changed each term. One of them had always a quasi-religious focus, such as Ethics, because we didn't do RE in the 6th form.
I don't remember much about the differences in the exam, except that there were several papers for the A level, which included a translation paper. With hindsight, I should have been OK with that, as I did language A levels.
However, I managed to get into the Uni and onto the course I wanted, despite not getting the required grades from my other A levels, so 42 years on it really doesn't matter that much.
I have a feeling that some Universities (not just Oxbridge) were a bit snooty about accepting a good GS A level grade in place of one of your main subjects.