About Gaius' magic, I don't think it's been specifically said. I think when Gaius was talking to Merlin about magic Merlin asked how he knew about it and Gaius completely deflected the question. So it was kind of implied, but not heavily. Also, in this world most people that do magic study it, but Gaius said Merlin is a warlock, meaning he was born with it as part of him. And I totally agree about Uther!