Sure, but there's a meaningful difference between "left leaning" and "left biased", is there not?
It seems entirely possible for a person sympathetic to the left to create content which is unbiased, or even biased in the other direction, based on external pressures.
The claim made wasn't that most journalists lean left, which may or may not be true (I think your assessment is probably a decent first approximation); but that "most media and journalists have a left bias", which is so far from being true in the UK that I suspect that GGP may not be arguing with facts in mind.
I read it as their inherent unconscious bias will be left leaning, and they will attempt to adjust for that in their output (certainly I've spoken to journalists based in Israel, and their views over a beer in the American Colony are very different to the considered ones they say on air)
It seems entirely possible for a person sympathetic to the left to create content which is unbiased, or even biased in the other direction, based on external pressures.
The claim made wasn't that most journalists lean left, which may or may not be true (I think your assessment is probably a decent first approximation); but that "most media and journalists have a left bias", which is so far from being true in the UK that I suspect that GGP may not be arguing with facts in mind.