The Girona Weather is very mild, slightly cooler than here in Marbella where I live, and with slightly colder winters.
The best times to visit are May, June and September, wonderful temperatures and mostly clear skies.
Clothes that you require
If you are coming here for a break between September and May bring a coat, jacket and jumper for the nights and just in case there are some cold days. This is particularly relevant in November, December, January and February when you will also need to bring winter coats.
If you are from the UK or Northern Europe you will be used to central heating and you may find the apartment or hotel where you are staying chilly in the evening, the best way to combat this is to wear slippers!
In the summer bring items to keep you cool!
Sunbathing is not really possible all year round especially between October and May can get quite cold winds. However if its sunny, its warm enough to sit outside, at least for a couple of hours each day for eating al fresco or just enjoying the warmth over a coffee.
You can sunbathe most days from June to the beginning of October.
Aircon in the car
You should definitely consider aircon from April until October, take a view on it from November until February. Even though you are unlikely to use the Air con much in the winter months the weather in Girona can produce the odd warm day in the winter but lowering the window down is generally all that is needed.