Multiple media outlets have reported the camera that caught the video is the property of a Nathalie Rollandin from France. Rollandin describes her experience with the seagull in the description of the video, uploaded to YouTube Sept. 2: "Funny video of how a seagull stole my GoPro, flew over the water in front of Golden Gate Bridge and how the camera incredibly managed to survive, recording everything!"
Either animals have a liking for GoPro cameras, or this is a hoax. Just over a year ago, another seagull in Cannes, France, supposedly filmed a different scenic and highly viral video, uploaded on June 23, 2011, with the exact same brand of camera before also depositing that camera in a retrievable location.
On May 23, 2011, one month before last summer's GoPro-carrying seagull video, another supposed GoPro owner posted a video in which a lioness picks up the camera with her mouth and carries it through the bush. That video now has over 400,000 views and includes information on where to buy GoPros in its description.
Another popular video, posted in August, shows what is purported by the uploader to be GoPro footage of a pod of actual, and not CGI, dolphins. (Source)
I decided to see for myself and I turned to Google again. This time I googled for "Animal picks up Canon camera," then I tried Nikon, Sony, Samsung and Olympus. No hits. However, when I tried GoPro several of them popped up.
In this case I agree. It has to be viral marketing. This is planned and executed to deceive people into thinking that they are watching people's amazing videos when it's actually a commercial.