They do have things to gain - credibility with the Arab and Sunni Muslim world for fighting against Israel and perhaps defeating it and excuse to operate their proxies and allies. Not to mention they are true believers, and even if they weren't, it doesn't have to be fully rational from your perspective; just from their own.

As for the Jews in Iran, there used to be 150,000 Jews in Iran before the revolution today there are ~9,000. Furthermore IRGC are very much the ideological inheritors of the type of folks who did the Allahdad (God's Justice) Massacre in Mashhad in 1839.