I am glad you are considering the 1966 Full Calendar as your next acquisition, as simply put, it is just a sublime watch.
As you very well noted in your post, Daos's superb review of the watch tackles some of the doubt you mention in your post, specially in regards to history of the watch and some technical aspects of the GP3300 caliber.
I am going to include the link once again, not only for easier reference for yourself, but for other people who might stumble upon this post, and as a result, develop a liking for the watch.
Now, on to your questions.
Yes, the GP3300 caliber is made entirely in house, and it serves as the base caliber for several watches in the GP lineup. To this base caliber, GP then incorporates the chrono module, power reserve module, et al.
This particular watch is an evolution of the Classique 1966 line, which is a hommage to the 1966 Chronometer which won the Neuchatel Chronometric Trials, if memory serves me correctly, and which was launched in 2006 with the time and date version. The full calendar is just another evolution of the watch which now also includes a superb equation of time watch.
I normally avoid making comparisons with other brands as they all have merits and have the arguments to occupy the place in which they are. We all have our values and perceive things differently. With that in consideration I will say that GP is an honest watchmaker with a long heritage and history, which also turns out, is linked to the progress of La Chaux de Fonds.
In terms of quality and technology they utilize, and innovations they produce, they are top notch. I am including three links below these lines that will take you to a review I made last year when I had the opportunity to visit the Manufacture. I am sure they will help you a lot in being drawned even more into the allure and charm of Girard Perregaux.
And, finally, you ask if they supply their movements to other brands? Indeed they do, and you might be surprised to whom they supply, one being Patek Philippe (for their quartz watches, I believe). Cartier, MB&F, Gerald Genta are also among the brands who rely on GP for the base movements in some of their watches.
I hope this info helps and please keep us posted on what you decide in the end.
Kind regards,
Alex
