G.K. (name unknown)
Höhr, Hesse-Nassau, Prussia. (now Höhr-Grenzhausen, Rhineland-Palatinate).
Entwerfer & Modelleur / Designer and Modeller
Known dates: Active 1890 - 1915.
Several names have been put forward for the owner of the initials G.K, which include - Gustav Kühne & Karl Girmscheid.
These are speculation only. No firm evidence has been offered to prove any of them correct.
Designed for:
Matthias Girmscheid: #42, #85, #92, #93, #100, #119, #155, #156, #157, #158, #170, #171, #172, #180, #181, #188, #189, #190, #402, #488/2, #518, #858, #887, #937, #941, #942, #974, #980, #983, #1004, #1005, #1010, #1012, #1014, #1016, #1020, #1024, #1026, #1032, #1631, #X3, #X15, #X16, #X17, #X18, #X30,
Reinhold Hanke #1388, #1631,
Villeroy & Boch - Mettlach #2384/1035 & 1036
Roßkopf & Gerz, #328, #332, #339, #489, #595, #650
Reference:
Reinhold Hanke. #1388, featured stein designed by G.K. 
Reinhold Hanke. #1631, designed by G.K. 
Impressed stein on Girmscheid #1032, also signed KB by Anton Kilian "Karl" Beuler. 
Another design by G.K. on unknown stein #7905? 
Reinhold Hanke. #1388, designed by G.K. 
Roßkopf und Gerz # 489 
Reinhold Hanke. #1388, designed by G.K. 
Impressed mark on Matthias Girmscheid #42 