The actress, 29, who has previously starred in Cutting It and Doctor Who, has been cast as the "fiery and vivacious" Kirsty Clements.
Gaskell's first appearance in the long-running BBC One medical drama will air on May 15.
"Joining Casualty has all been a great big whirlwind, but the cast and crew have been welcoming and friendly from the moment I walked through the doors," she said.
"It's been really easy to fit in and I've not once felt like the 'new girl'."
Series producer Oliver Kent added: "We are delighted to welcome Lucy to Casualty. She's exactly the right person to bring the human whirlwind that is Kirsty to life and she's settled into the Casualty team beautifully."