The mechanic - played by Danny Miller - appears in court late this month, after he was charged for his part in the assisted suicide of his paraplegic boyfriend Jackson Walsh (Marc Silcock).
Gaddas, best known for his role as Larkhall prison governor Neil Grayling, previously appeared in Coronation Street as Vinny Sorrell, who had an on-off relationship with Rovers Return landlady Natalie Barnes (Denise Welch).
"I was delighted to be approached to appear in these episodes and really moved by the performances," the 51-year-old actor told the show's official website. "The scenes in the courtroom are very hard hitting and it was a privilege to be involved."
While Gaddas takes on the role of the prosecutor, Benidorm actor Tony Maudsley will play Aaron's lawyer. Meanwhile, Mary Jo Randle - whose credits include The Bill, Wire in the Blood and Silent Witness - will appear as the judge.
Miller acknowledged the guest cast for their contributions to the "brilliant" episodes, telling Digital Spy: "We've got some great guest actors in to play the defence and prosecution. They were fantastic."
> Read our full chat with Emmerdale actor Danny Miller