But GEP's quite often don't look at A-levels. Some may be dependant on your GAMSAT / MSAT scores whether or not you even get an interview.
There's no guarantee of being accepted onto any course whether its GEP or 5/6 year. How many straight A students have been rejected for medicine? Loads!
Do what you think is best for you, since you could quite easily apply to 4 of the 5 year courses and get rejected from all of them whereas you could get accepted onto the GEP. Or vice versa.
The whole thing is a lottery anyway. Just make sure you have lots of work experience etc to strengthen your application.