There is no evidence that it's 100% that Gaudreau is out of shape, but it wouldn't surprise me.
He looks a bit slow and lethargic on the ice this year. Not creating separation as much as usual, skating is not as dynamic, and he is playing more slow.
Whether that's all mental or it's physical too is hard for us to tell but it's pretty clear he's not at his peak right now.
And I do think part of it is him just settling for outside shots this year - look at the teams shot map with him on the ice:
That is a lot of low percentage shots outside the danger area from his side of the ice.
I can't share the last year view since it's restricted to subscribers but it's pretty much deep red right in front of the nice, and right up the middle of the ice. He's just settling for low percentage shots right now.