Probably a 1970´s modelDate: Nov 26, 2007,12:29 PM - (view entire thread)
the number in question is the reference code, which suggests a production date somewhere between late 1960´s and mid/late 1970´s.
The best place to have it serviced/restored most probably is GP Switzerland; for further help, i´d recommend to visit a regional/local GP dealer or the GP distributor in your region.
As you may notice, that´s a time when most GP were not completely comparable with what the brand does today.
That´s reflected by the value of most (not all, but exceptions are a bit on the rare side) of these pieces and particularly if the watch is in need of full restoration including exchange of many parts, it may not be justified unless the watch in question is very special for you.
There´s no easy way to guess the estimate of possible repair costs, as these do vary depending on the work needed to be done. Of course a "simple" service is likewise affordable (usually below US-$ 500), but watches that went through a tough life may require to reproduce parts for the movement, dials/hands or case parts. Doing such on a one-off base for specific watch only is pretty expensive to do, therefore it really needs to have a qualified inspection. But once you send the watch in, you´ll receive an estimate of approximate repair costs.
Greetings from Germany,