[GC 17: 2]
Presa fu la città dal Re d'Eggitto,
Con altre molte, in lacrimosa guerra
Quando a l'Imperio già de' turchi afflitto
Tolse ei gran parte de la siria terra
Insino a Laodicea (sì com'è scritto),
Che d'alte mura s'incorona e serra;
Ma Gaza parve più opportuna parte
Da raccor varie genti e schiere sparte.
The city [of Gaza] was conquered by the King of Egypt, together with many others, in a distressing war when he took a great part of the Syrian land away from the afflicted Turkish Empire -- up to Laodicea (as it is written), crowned and enclosed by high walls. But Gaza [among the other conquered cities] seemed the most fitting place where to gather all those different peoples and armies.
Tasso and/or his sources make some historical mess. Gaza was conquered in 634 by Caliph Umar, who however was not the "King of Egypt," and did not take away the city from the (then non-existent) "Turkish Empire" but from Christian Byzantium. Egypt would be ruled by the Arabs starting from the year 641, therefore later than Palestine.
Moreover, in the time period in which the poem is set, i.e. the end of the First Crusade in 1099, Gaza experienced a phase of decadence.
The word "Syria," as was often the case during the Middle Ages and the Renaissance, means the Middle East in general.