Fan Theory why Sauron didn't just kill Gandolf (Movie Canonical)
So this applies to the movies as thier own story\ event line. When Gandolf challenged Sauron in Dol Gudor and lost, it always kind of niggled at me as to why he didn't just kill him instead of putting him away in the gibbet.
I could understand Saruman not wanting to kill him as despite their differences they were still bound by years of brotherhood. Also Sauron repuditly had a penchant of killing wizards, I remember a codex reference in Shadow of Mordor about the Dark Hand bringing the heads of two Istari (Wizards) to Sauron as a gift.
So with that in mind, the only reason I could think of Sauron sparing Gandolf is with his great power and insight (Sauron's) he may have known it was Gandolf the Grey's fate to fall and be reborn as the much more powerful Gandolf the White. Not wanting to have that come about he could have decided it wiser to just take the wizard off the playing board so to speak by imprisoning him.
- Currently watch classic Cinema like Gods of Egypt