The actress will depart the sci-fi drama midway through its seventh series, along with her co-star Arthur Darvill (Rory).
Gillan told IGN: "I'm not allowed to say [what happens]. But I know [the episodes are] going to be damn good. It's so exciting.
"I think it's the best ever. I can't say anything, but oh God, I'm dying to say something!"
Gillan confirmed that she had planned her exit far in advance with Doctor Who showrunner Steven Moffat.
"I called Steven Moffat and... then basically told him roughly when I wanted to go," she explained. "He told me where the story was at and where it was going and then we kind of together came up with it."
Gillan added that she wants Amy to "go on a high" when the character is "at her prime".
"Steven Moffat comes up with endless, amazing ideas anyway, but I wanted to make sure that I went on a high," she said. "There's just something quite nice and appealing about that to me. I don't know, it just felt right! I like to go on instinct."
Doctor Who will return to BBC One in the autumn.
> Doctor Who to resume filming this month, Saul Metzstein to direct
Watch Karen Gillan discuss her Doctor Who exit below: