The writer told Premium Hollywood that it was "exciting" to have the Harry Potter star on-set.
"He's brilliant," said Moffat. "Of course he's brilliant. Absolutely brilliant. What a voice.”
The showrunner added that Gambon was not a huge fan of the series prior to accepting the part.
“I didn't get the impression that he was a fan of Doctor Who, except insofar as everyone in Britain is at the moment," he explained. "[But] if it's a good script [and] they're being offered prime-time on Christmas day... there's a real chance you can get anyone for that."
It was previously announced that the episode's plot would involve "a clever twist" on the Charles Dickens story A Christmas Carol.
The currently untitled special will air this Christmas on BBC One.