"It's also smaller and weighs less than the gray squirrel, which gives the gray squirrel a competitive advantage. It's why it's displacing the red squirrel in some regions of Europe. There's also other advantages that gray squirrels have, for example they can digest food more efficiently etc.

It's natural selection in action!"

Actually it's not.  The Grey squirrel is indigenous to North America "naturally" it never would have gotten to Europe.