Parking is a total nightmare in both Bethesda and Upper Gerlan, the roads are so narrow, with hardly any parking spaces available, and definately residents only.
I cannot remember who told me about the car park just behind the George, but ive used it for many years, even when living in South Wales.
There is a walk of around a mile or so, until you reach the main mountainside of the Carneddau, but for peace of mind, and knowing your car is safe, its the best location.
The main criteria, is that its totally safe, is free, for as long as you want.
Some years ago, i parked there for nearly twelve hours, and the car was as i had left it, which is not alway's the case nowdays