Here you go. From Wikipedia.
The siege of Sevastopol was the gun's first combat test. Installation began in early May, and by 5 June the gun was ready to fire. The following targets were engaged:
5 June
Coastal guns at a range of 25,000 m. Eight shells fired.
Fort Stalin. Six shells fired.
6 June
Fort Molotov. Seven shells fired.
"White Cliff" aka "Ammunition Mountain": an undersea ammunition magazine in Severnaya ("Northern") Bay. The magazine was sited 30 metres under the sea with at least 10 metres of concrete protection. After nine shells were fired, the magazine was ruined and one of the boats in the bay sunk.[4]
7 June
Firing in support of an infantry attack on Südwestspitze, an outlying fortification. Seven shells fired.
11 June
Fort Siberia. Five shells fired.
17 June
Fort Maxim Gorki and its coastal battery. Five shells fired.
By the end of the siege on 4 July the city of Sevastopol lay in ruins, and 30,000 tons of artillery ammunition had been fired. Gustav had fired 48 rounds and worn out its original barrel, which had already fired around 250 rounds during testing and development. The gun was fitted with the spare barrel and the original was sent back to Krupp's factory in Essen for relining.
The gun was then dismantled and moved to the northern part of the eastern front, where an attack was planned on Leningrad. The gun was placed 30 km from the city near the railway station of Taizy. The gun was fully operational when the attack was cancelled. The gun then spent the winter of 1942/43 near Leningrad.
The gun appears to have been destroyed to prevent its capture some time before 22 April 1945, when its ruins were discovered in a forest 15 kilometres (9.3 mi) north of Auerbach about 50 kilometres (31 mi) southwest of Chemnitz.
So yeah, 48 rounds fired.