If you recall, Italy surrendered to the Allies in December 9, 1943 (turn 79). Anyway, even before the conquest of Sicily, Western Allies had already sent transports to mainland Italy coasts, so in turn 80 (December 12,1943) the Allies landed and took unopposed the port of Taranto.
In the new year, more and more units landed. Although at a slow pace, Allies approached to main German army defensive line that, expectedly, was built along the Gustav Line. Germans also tried to kept Naples in their hands and so a Corp unit was deployed to guard the city.
The seizure of Naples would become the most immediate objective for the Allies and this first offensive operation was met with success in March 18, 1944 (turn 84). With ample air support, British army, firstly, got good rolls when attacking the city and, ultimately, a Canadian landing 1xS of the city, meant to finish off defending German corps.
All of this time, Wehrmacht made no counterattacks at all. So Allies started to invest Gustav Line itself. Referred defensive line had been continuously bombed by Alied naval and air units, German paratroopers getting significant suppression.
First assault to one of the forts came in turn 86 (April 27, 1944) to which the image below corresponds.

Shot is taken before Allies moves. In Allies turn, British attacked LXXXVII AK, located 2xSE of Rome. After having been heavily bombed by RAF TAC's, referred German HQ corps (Kesselring) took a beating but held the position.
Allies would try again to breach Gustav Line 2 turns later. Attacked German units, although depleted, held their positions once more. No matter the potent Allied attacks, with the Germans continuously swapping their depleted units with fresh ones, things were closer to a stalemate than to an immediate Allied breakthrough.