The 50-year-old actress has acknowledged that she'll move into a "a pretty obvious age bracket" when she allows her hair to go grey, and she's admitted that it'll be an important moment in her career.
She said: "There will be a point in my career where I make an active decision to allow my hair to be grey.
"That is a big decision, because all of a sudden it means one is being cast from that point on in a pretty obvious age bracket."
The actress - who is, perhaps, best-known for her starring role in 'The X-Files' - conceded that ageing is something she simply has to "come to terms with".
Speaking to the BBC, she explained: "There have definitely been moments where I've been in a new show or film and been the oldest person there.
"No matter what our profession is, it's something all of us will have to come to terms with."
Earlier this month, Gillian admitted it was "arresting" to see how her appearance has changed during her career.
However, she is now philosophical about the consequences of ageing.
She shared: "If you watch yourself on film, there is a certain point where you see yourself change. It's arresting.
"It can either be completely traumatic or something that instigates a shift of consciousness towards thinking about what's important. But you have to go through that trauma first, to mourn."
The actress now accepts she can't do anything to halt the ageing process and is, therefore, determined to "celebrate" it.
She said: "The thing to remember is, how one looks in the mirror is the youngest one will ever look again. So you can't do anything but celebrate it!
"Ageing is something we all need to find a way to embrace - the inevitability of age, of decline, of ... rot."