The crew who operated these Panther-turret bunkers were specially trained men who formed part of a Festungs (Panther-Turm) Kompanie, normally made up of 12 turrets. The turrets of the Hitler Line in the Liri Valley (later known as Panther Turm Kp 1.), were initially attached to the 15th Panzergrenadier Division, 10th Army, but when this division was relieved Kampfgruppe Straffner absorbed the Panther turret company. The 1st Fallschirmjager Division manned other turrets along the line, particularly around Piedmonte near Monte Cassino. A sister unit, Panther Turm Kp. 2, was established by the 14th Army to man positions on the west end of the Green Line and the central passes. Later in the war, companies 1209 and 1210 were also added.
Normally, the majority of the defensive positions were manned by ordinary artillerymen, grenadiers, infantrymen or quasi infantry like the Fallschirmjager. The Panther turret crews would be selected from these units and trained as Festung Kompanies.
An excellent and inexpensive reference for these installations is Osprey's German Defences in Italy in WW II, from which I paraphrased this info. Several photos plus a two page color illustration with cut-away view of a Panther Turm are included.