Yes, GP had problems with the after market service.
Last few years GP worked hard to improve the service line!
Is it flawless? No, but they work hard to improve.
But as for all manufacture this is a multi level issue.
If not a brand boutique, then the local AD watchmaker has to be certified for the specific calibre to be allowed to do the service locally.
(Yes, most watchmakers would be able to preform service without a special course/certification for each calibre, but it is a way for the brands to ensure the service quality).
Service parts, it is a well-known fact that most brands has stopped to send parts to a third party for usage. We as end customers may think this is bad, but again a multi level issue. Often we think that the brand name on the dial is responsible for any problem with our watches, even if the service was preformed by someone else. By limit the supply-line, brands try to take control to make sure service is made in a professional way.
In this specific case as it is a vintage service, the wait is longer. Why? Because all watchmakers are not able to preform a vintage restoration. During the last few years, the vintage workshop at GP had an increased request for service.
I guess that we here in the forum had a part in that ...!
Best
Blomman