In an interview with InnerSPACE, Gaiman insisted that he never doubts Smith's portrayal of The Doctor's wisdom and world-weariness.
"I think Matt does something that nobody else has ever done in that his Doctor feels old," the scribe explained.
The writer, who is perhaps most widely known for writing the Sandman comic series, further asserted that the actor's portrayal is even more impressive given that he is the youngest star to ever take on the iconic role.
He continued: "I love the fact that you [can] look at the press that came out when Matt took over the role and people were saying, 'Matt's going to be too young'… Actually he plays [it so it's] the first time you're always aware that this is a 900-year-old Time Lord.
"This is somebody who is really, really old and he's been [travelling] for a long time. He's not thinking quite like we think and, yes, he's alien, but more than that he's old."
Before Smith made his screen debut as The Doctor at age 27 last year in 'The End of Time, Part Two', Peter Davison had held the record for the youngest actor to take on the role when he joined Doctor Who in 1981 at 29 years old.
Gaiman recently revealed that writing the upcoming Doctor Who episode 'The Doctor's Wife' made him feel "the nearest to being God that [he has] ever been or will ever get".
Doctor Who will premiere on April 23 on BBC One in the UK and BBC America in the US.
> 'Doctor Who' - Everything we know so far!
Watch Neil Gaiman discuss his experience writing for Doctor Who below: