The model GK was merely a stock and finish shape of the post-1952 Mannlicher – Schoenauer, made to European tastes. GKs share all the metal work with the "American" models MC and MCA, though the bolts are blued on GKs, polished bright on the others. Late versions of all three "models" have wimpy, sheet metal tang safeties instead of the earlier side safeties. IMHO "GK" stnds for Gerader Kolben = straight buttstock. This stock shape is the distinguishing feature of the GK. Model GK rifles in 7x64 are now the most often encountered Mannlicher – Schoenauers here in Germany. The GK designation is so common here that Germans often call any old split bridge, true Mannlicher-Schoenauer a "GK" to distinguish it from earlier and later Steyr products. The MC and MCA stock variations were not even offered in Germany.
The model NO is quite confusing. Apparently Steyr called all sorts of special order, not catalogued versions NO. So NOs come in many different configurations, some with straight bolt handles when bent ones were standard, some with bent ones.
Here are (not so good) pics of three NO marked rifles of entirely different configurations:
A full – stocked rifle with a 60 cm = 24" barrel in 9.3x62. Note the bent bolt handle and the late type tang safety.

Two special rifles in .308 Win aka 7.62 NATO, specially made when Biathlon was still a largely military sport, shot with full – bore rifles , one with a folding stock (from the Steyr factory book), the other with a straight GK-type buttstock.: