IIRC better part of the Persian army took a hike. One minor shortcoming in the Philippo-Alexandrian tactical system was that the majority of the troops weren't too hot for envelopement and pursuit - rather, Alex generally had to be pretty darn clever to keep his phalanx from getting enveloped while the corps de rupture heavy cavalry were busy deciding the fight. Phalangites were for fairly obvious reasons el mucho suckage for chasing anyone, so the pursuit was very much the job of the cavalry.