'Type XX'-type watch by GP!
I inquired about a similar but not quite the same watch in December last year and got help from one very kind fellow collector. He could not quite help me to track the history of my watch but he could document the history of your watch. What I learned was that your watch was part from GP late 60's collection, as illustrated in at least one of their catalogue from that period.
Mine, which is similar in design, is signed 3 times (dial/ inside case/ movement) plus an additional GP monogram on the movement. It has the very rare Valjoux cal. 720 with 'retour en vol/ flyback' chronograph function, as to be expected from a 'type XX' type of watch. Its case is particularly thick to accommodate that flyback function. I do not know whether yours is also a flyback. There is another marked difference with your watch regarding the 3 o'clock minutes' sub counter. The pity is that GP, whose archives are far from being complete, could not help. A real shame...
I' hope that you will get more answers from the GP experts on this forum.
Here is a picture of my watch that I posted earlier on Watchprosite. I absolutely adore this watch.