First off, UCAS should be able to answer your questions entirely. Be patient and read through all the information on eligibility to apply. You can also go to each universities website and read about eligibility for the programmes. It will take you time. But so will being a medic. So just spend an evening and read through each of the application requirements. Oh, and the A100 thing: that's the course code that UCAS applies to courses so they know what you're applying for. Medicine is usually A100 or A101 or something like that. Again, this is found on the UCAS website or on the university's website under 'How to Apply'.
Right, here's the skinny on GEPs: Most GEPs are designed for British/European nationals who already hold an undergraduate degree. The second advantage (after being a four year programme, versus the traditional five year programm found at all other medical schools) is the students typically only pay tuition for the first year of the four year programme. The NHS then pays the tuition for the remaining 3 years. Free medical school? Is there a catch? Yes! Of course there is: Unless you've lived in the UK or Europe for three years before starting the course you would not likely be considered for the GEP as the preference is given to those who will stay here in the long term AND they are the only ones who would qualify for their tuition fees to be paid by the state.
I know in the past Oxford and Cambridge have had spots on their GEPs for non-UK/EU residents. They cost a lot (25,000GBP/year) and I think there were only 12 spots available. They get something like 1200 applications. I suppose it's worth a shot, but bear in mind that it'll be 1 in every 100 who gets in.
But what if you're a dual national? (i.e. a Canadian with a British Passport or an Italian Passport). Short Answer: Doesn't matter unless you can prove you lived here (or somewhere in the EU) for 3 years before the start of your degree. They're ruthless. They'll check.
UKCAT: Everywhere requires this test to be completed prior to application with the exception of Bristol and Liverpool (double check Liverpool)
Your grades are good - as is your education and experience. Consider applying to the 5 year programmes as well if you're really serious about studying medicine. Remember, you can only apply to four options on the UCAS application. Make them count!
Best of luck