I agree with both sides here. +/- can be used to make a point if you wish, but it's so easily affected by other factors such as matchups and linemates that even in the most relevant context- comparing players from the same team- it's tough to show anything without a healthy dose of salt.
The team was a defensive trainwreck early in the year, and Giordano was on the ice for half the game, with goals going in consistently. However, Giordano's +/- wasn't suffering because he was causing goals against by himself, it was struggling because every five-man unit was struggling every single shift, and he was on for half of them (the half with the tougher matchups). Without question, Giordano also was playing less than stellar hockey early on, but he's playing better now and lo, the team has improved with him. Gaudreau has been providing offence all season with the offensive opportunity given to him, and that's been able to mitigate any damage in terms of minuses from early in the year when the team was a defensive trainwreck. It seems like Gaudreau's line gets to play with Hamilton's pairing more than Giordano's pairing lately as Giordano is tasked with shutting down the other team's stars.
At the end of the day, they're both great players currently playing great hockey. +/- isn't going to tell you anything more than confirming that Gaudreau definitely generates offence for his team at an elite level.