I agree, a very nice rifle, - the barrel was made by FN using a semi-finished barrel blank. This steel firm is not known with certainty, but it is fairly commonly encountered.
The "f" recoil, and "C" stock are subcontractors as Mauser99 stated, both Saxon firms, "f" from Leipzig and "C" an office table/desk maker in Dresden.
Gustloff Weimar, was an assembler, they made very few parts on the rifles they assembled. They did make some stocks and some barrels but they are rarely encountered.
Like Steyr, Gustloff's rifle production was a sideline, and insignificant aspect of the company overall. Both were part of a much larger corporation, and both were essentially givernment corporations (with a facade of a privately owned enterprise)
No way to say with certainty which branch of service the rifle served, though the Army/Heer would be by far the most likely. Most production went to them- that is why the ss-scumbags restrained their industrial involvement in the early war period. As whether you produced the rifle or provided the labor, distribution and acceptance was an Army affair and not until the later period did this start to change (and you see more ss involvement in private enterprise)
In the early stages, Himmler himself stopped ss overtures to buy up small arms factories for this reason. His resources were best applied in the occupied territories- Poland and the Protectorate, where they had a great deal of influence over industries (Brunn actually did weapons research for the ss!).
If you can, try and show us the full barrel coding?