1-3, 7, 10-12: GEM tends to be statistically more competitive than standard entry medicine for undergraduates, as there are fewer places and competition is very high for these. You will need to actually do your own research to identify the universities offering GEM and what their requirements are. This varies between courses. Some require UCAT, some require GAMSAT. Exactly what area your degree needs to be in (e.g. bioscience degree, science degree, any degree) and the expected level from that (usually a 2:1 or above but some may have routes for those with a 2:2 and e.g. a PhD on top of it, although I imagine many graduates will be on track for a 1st). They will also have varied work experience requirements although invariably you will need at least some kind of experience similar to undergrad medicine for most, and some have more extensive requirements.
4: Discuss your motivations for medicine, reflect on your work experience etc. Not all medical schools even read the PS and most don't score it as part of shortlisting so it's not the most important thing in most cases I think.
5: For funding for an accelerated 4 year GEM course you get partial SFE funding for first year (full maintenance loan plus partial tuition fee loan - you pay the first ~£3000 or so of tuition fees yourself) then the remaining years you get NHS bursary funding (with possible reduced maintenance loan top up to your bursary if eligible for higher means tested amounts). For a standard entry (5-6 year) course you would only get a maintenance loan and no tuition fee loan for years 1-4, and then get NHS funding year(s) 5(-6).
6: This is purely a personal choice.
8: Not relevant to applying to medicine but valuable if you choose not to go that route.
9: 4 years GEM (or 5-6 years standard med) plus 2 years foundation and thereafter depends on the specialty. Physicianly specialties normally have 2-3 years IMT before commencing specialty training, surgical specialties usually have 2 year CST first, but there are variations, different routes through (e.g. doing CST or IMT then switching to e.g. GP). Also some specialties across the range have run through training from the start. You'll be working as a doctor from when you finish medical school though, but you'll be working across the range of medicine for at least 2 years so you shouldn't be overly focused on specialty choice before even applying to medicine.
A lot of these questions are ones that would be answered simply by doing some research into what GEM involves generally and checking the requirements on the uni webpages which is a necessary part of applying to medicine (for standard entry or otherwise) anyway.