A GS 1220 Club. The plate puts it in the 1975-1976 range. The cancer on the trunklid and the numerous dents are a testament to the hard life this car has probably had. I believe that blue color to be original, though it's obviously faded.
The GS was introduced in 1970 and was designed to slot in between the smaller Ami8 and the DS. It won the European Car of the Year for 1971. It was powered by a series of anemic, though simple, air cooled flat 4s. The exception to that was GS Birotor which featured a rotary engine developed by Citroen and Wankel. Less than 1,000 of them were built and they are very rare today (see Avignon photos for one).
Thanks to the Hydropneumatic suspension that was made famous by the DS, it had a better ride than any car in its class and ended up selling quite well. It was replaced by the GSA in 1979 but it was essentially identical to the GS. The body was the same but it now had a hatchback. Other minor differences include taillights, wheels, and bumpers.
While they're by no means common, they're not rare either and they are unfortunately not worth very much so a decent example can be had for around €1,000-€1,500.
A personal note-- I own a 2CV and a GSA and my cousin owns a DS. The GS drives like a faster 2CV and rides like a DS, it's the best of both worlds.