It's an early German Dagor before Goerz adopted the name which is short for Doppel Anastigmat Goerz, and it also uses an old style aperture nomenclature. It's an early Dagor they were introduced in 1892 and by the early 1900's the 300mm/12" had been redesigned to give f6.8.

The 6 is f7.7
12 = f11
24 = f16
48 = f22
96 = f32
192 = f45
384 = f64
768 = f90

Coverage is good, 10x8 at full aperture and 10x12when stopped dow to f16 and 12x15 by f64, and depending on the condition should be a good performer.

It needs cleaning with care, but may be a very usable lens, you need to find a shutter, a Packard or Thornton Pickard ideally to use it with. You can find plenty about Dagors on the internet.