Gunn had initially denied the rumours that Fillion, 43, would star in the film, but later confirmed the news, saying he has a cameo in the film, reported Aceshowbiz.
"@nathanfillion has a very small fun cameo he did as a favour to me," Gunn tweeted.
Fillion, in April, had hinted at his possible appearance in 'Guardians of the Galaxy'.
Asked if he wanted a part in the Marvel superhero movie, he had replied, "Wanting to get a part....Or maybe did."
"Maybe you'll be surprised. Maybe. Check the credits," he had said.