I'd put the basic point to them and see how they answer it:
Dear Gwynedd Council,
I dispute liability for PCN GW33544579 on the basis of inadequate signage. I was parked on a road and I did not see any signs, let alone any compliant signs, indicating that any sort of parking restrictions are in force.
While it is the responsibility of the motorist to check for signs and restrictions before they park and leave their vehicle, it is the responsibility of the highway authority to ensure its signs are compliant with the Traffic Signs Regulations 2016, and also that the signage as a whole is adequate within the meaning of regulation 18(1) of The Local Authorities' Traffic Orders (Procedure) (England and Wales) Regulations 1996.
The signage in this instance is blatantly not complaint with the 2016 regulations, and it is certainly not adequate. It follows that the contravention did not occur and the penalty charge must be cancelled.
Yours faithfully,