As already remarked, it is not historically true but a detail fictionally added by Tasso that the Sultan of Egypt took part in the First Crusade. A symbolic figure, as he used to be "The" Sultan during the Middle Ages, but of course in Tasso's times the main and most dangerous Muslim center of power had shifted to Constantinople. So, the following portrait of the Sultan of Egypt mirrors the mixed feelings that 16th century Europe harbored towards the Turks.
[GC 17: 11]
Ancor guerreggia per ministri, et have
Tanto vigor di mente e di parole
Che de la monarchia la soma grave
Non sembra a lui soverchia mole.
Sparsa in minuti regni, l'Africa pave
Tutta al suo nome, e 'l remoto Indo il cole;
E gli porge altri volontario aiuto
D'armate genti, ed altri ampio tributo.
He still wages wars through his ministers,
Having so much strength in his mind and words
That the heavy weight of monarchy does not
Seem to be an excessive load to him.
The many small kingdoms of Africa
Tremble at his name, India worships him;
To him, some grant voluntary subsidies
In armed people; while others, large tributes. (*)
(*) There are some echoes from the "fabulous" descriptions of Solomon's kingdom in the Bible, see 1 Kings 5: 1, 14; or also from the legend of Prester John.