The other post-1952 M-S standard catalog models were named for their buttstock shape: MC = Monte Carlo, MCA = Monte Carlo Amerikanisch. These models had the same bent bolt handles too. So there is no logic in naming the GK for the bent bolt handle instead of the stock shape like the other contemporary catalog models.
I don't know what the meaning of "Modell NO" is, maybe "nicht ordinär". I have seen several NO marked Mannlicher-Schoenauers, all post-1952. The only thing they have in common: Their configurations are listed in no Steyr catalog. Some examples: One is a late post-52 action with secondary sliding tang safety, 24" rifle length barrel in 9.3x62, buttstock like a GK, but full stocked to the muzzle like a Stutzen.

Two others rifles with post-52 receivers with double pull military type triggers and straight bolt handles and elongated safety wings, without side- or tang safeties, with side-mounted wide sling swivels, mounted with much altered Anschütz target peep sights. Marked "Modell NO / Kal. 7,62x51". These were made in small numbers, perhaps two dozen, in the 1960s for the military biathlon teams of both the Austrian and German armies.

Another a rifle in 9.3x62 with post-52 receiver, straight 1950 type bolt handle and without any secondary safety. So IMHO the NO model designation was for special order, custom variations or special order chamberings that did not conform to the contemporary catalog models.