My interpretation is
GX828648, but I'm just a little bit uncertain about the penultimate digit. (The photo isn't great but it's not really easier to see in real life.) Do you agree with me that it looks like a 4?
Good luck - there's a very high probability noone noted the serial number, or registered it anywhere that you can search. Giant themselves might be able to tell you what shop sold it, but whether they kept records or not, who knows.
Your other option is to report it to the local police station as "found property" and after a month if the owner doesn't claim it then its yours legally. We scored a nice tent that way that blew into our yard once, fully assembled but never pegged to the ground.
You can assume its probably not repainted, so is a Blue bike from new.
I'd agree with your reading of the digits. Possibly the first character is a C or a Q or maybe an O, but the rest look exactly right. G for Giant might make a lot of sense too.