Lots of comedians have a similar gimmick. Steve Carell is great at it, which is probably why he was so good at the same role that Ricky had on The Office. Norm MacDonald was famous for being too smart and hiding it, but he did it slightly differently.

Maybe the closest other person I can think of to Ricky Gervais's method is Zach Galifianakis.

As for why he always acts the same way, that's exceptionally common for comedians. They just turn some switch to "on" and that's how they act. For an extreme example, think Gilbert Gottfried. He always acted the same, with the same voice, whether it was a role or just him hanging out, but it wasn't his normal voice. He did that voice whenever he was making any appearance. If you look, you can find occasional clips of him using his normal voice. There's one from the Howard Stern show where he left them a voicemail.

I think most comedians, when they turn that switch to "on", they just become a more extreme version of their own personality.

Gervais didn't start out pursing comedy as a career. He got into it rather late. And I think that strongly affects his persona, as well.

What is even the difference between a stupid man and a clever man method acting an idiot?

For a comedian, it's pretty easy to tell if you're paying attention. The clever man will think of something funny that other people wouldn't think of, but the content may be stupid. The stupid man will say stupid jokes that everybody else was thinking at the time.