EN 10084 is the standard for case hardening steels that references grade 16MnCr5 (1.7131) The following are the symbols that can be used for specifying the ordering condition:
+U = untreated
+S = treated to improve shearability
+A = annealed to maximum hardness requurements
+TH = treated to hardness range
+FP = treated to achieve ferrite-pearlite structure & hardness range
+HW = as hot-worked
+PI = HW + pickled
+BC = HW + blast cleaned
+ H = normal hardenability
+HH = restricted hardenability (high)
+HL = restricted hardenability (low)
The letter G does not appear in any designation in this standard. Perhaps another standard references this designation (old DIN standard for steel or heat treating?). Can you provide any additional details that may be helpful?