The Guardian definitely isn't centre: it's centre-left. I read it online every day and I believe it's by and large the best journalism out there. Some of the writers - Rafael Behr, Barney Ronay, Nesrine Malik - leave me awestruck in admiration.
As you say, the paper carries content from right-wing writers - Henry Hill, of ConservativeHome for one. It sticks in the craw a little - I mean, it's not as if they don't have their own papers! - but I supposed it's a mannered 'Look, we review the whole spectrum of opinion' type conceit we can live with.