If you are on the Costa Blanca then you can register with torrevieja-salud.com as it covers quite a large area.
You will need your SIP card number and NIE (ignore the first digit of the SIP card) plus a Spanish mobile phone and register. They will send you a password and you can log on to Yo Salud. You then change the password the first time.
From there, you can send a message which will go straight to the GP you are registered with.
If you didn't have a pre-arranged appointment, then loads of people could turn up and you wouldn't really want to go to a surgery full of ill people at the moment. Hence the need for the appointment in advance.
Our surgery doesn't have a barricade of chairs but the doors are locked. Before they'll let you in you have to confirm the appointment has been made.