Gun Metal Grey is written by Lau Choi-wan and Leung Yan-tung, with Terry Tong serving as the executive producer.

The Chinese title of Gun Metal Grey literally means "criminal police", which can also be used to a describe a cop who commits a crime.

The English title is a wordplay on the colour of guns, a representation of criminal justice, and Felix Wong's character Stone Sir, a cop who finds himself trapped in a grey area of morality.

Stone was framed in a killing of a happy family, and convicted in jail for 15 years, before he saw a newspaper article which allowed him to overturn the case.

He eventually returned to the police force and was sent to Mad Sir's team (played by Michael Miu), a good brother of his during his time as a cop.

Although initial reception of the trailer was not as popularly received as the channel's other anniversary drama, No Regrets, many viewers expressed that Gun Metal Grey would be a success once it airs due to it being a "Five Tiger collaboration.

A music video of the drama's ending theme, Ngor Tik Lei Hoi Ya Si Oi (lit.