What is one rule or policy companies can implement to reduce gender bias at work?

For me I'd go with more women in key management positions. They consider women when making decisions, for example making more parent friendly choices. In my experience it helps with diversity overall, other groups are more likely to occur to a woman.

I'm not sure - I would probably want to look at what is well evidenced, i.e. what actually demonstrates reduced gender bias. Bias might be really hard to actually measure, so we might want to be specific about what we care about, e.g.
salary gap between men and women
% of workforce that is men vs women
and so on ...
Policies that I've seen and come to mind (but may be flawed still, I haven't thought much about this):
provide childcare at the workplace, have somewhere your kids can be safe and taken care of while you're at work
seek out new hires, interns, and other candidates from all-women's universities and colleges in addition to ivy leagues, major state schools, etc.
have strong maternity and paternity leave
- You make excellent points about the maternity and paternity policies. Fathers shouldn't be ignored, and also it puts the parenting responsibility on women, leading to managers being less likely to hire or promote them. Supporting parents is a great way to reduce gender bias.

Ensure it is illegal to ask about pregnancy.
Maternity leave that is at least 12 months.
These are both important because women are assumed to be 'less productive' due to childbirth and are therefore given less opportunity. Which amounts to a betrayal of future generations since children should be able to bond with mothers (and vice versa) during the first year of life without it affecting mom's oppurtunities. Supporting women is our investment in the future
Keep men away from each other. Studies have shown that, when men are concentrated in groups, testosterone and aggression go up while empathy goes down. Men then begin to assume they have dominion over all other living things, including women.
Men need to spend time with women and children to become more balanced individuals who prioritize empathy and inclusiveness. Many men would also benefit from having greater exposure to and respect for nature also.
Don't promote gender neutrality blindly. Too many institutions aim for 50/50 representation without giving women the tools they need to succeed, which emboldens those that wish to claim women are less capable. Support needs to be provided from the ground up to balance for historical injustices.
Probably the most obvious but.. Just listen to women. Men tend to not listen to or respect women when they are in that hypercompetitive mode driven by testosterone imbalance. Too many (especially Western) men worship at the alter of logic and reason while ignoring emotional intelligence or right brain development in general, at the expense of women and others.
Patriarchy fails both men and women in its own unique way so development on both sides is needed to create a more inclusive world.

The question of the gender should not be relevant at all when choosing a professional and the fact you insist on making it so will only aggravate the issues.
- While ignoring gender sounds good on the surface, I think by ignoring we default to the uncritical and unthinking status quo, which unfortunately is still gendered in nature, i.e. there is something called "implicit bias" where people have internalized biases and preferences that they are not even aware they are thinking or having.
  Gender blindness can still be a useful tool, for example, instead of attacking OP for asking how to reduce gender bias, you could have suggested a HR policy that removes any gendered aspects of candidates CVs or applications so that decisions on who gets interviewed can't be subject to those biases.
  
  Giving an opposite opinion of your own does not mean someone is attacking you, and it's sad that you feel that.
  At any rate, you are saying just what I said, only with different words. It seems that you prefer for it to be a bias, only for it to be in your favour. Which sounds only natural given we are all humans, but i fail to see how it'll solve anything. Putting incompetent people in power is just wrong.
  this thread should be suggesting the hr policies to ignore sex instead of favouring a specific sex. What's wrong with going with merit?

- What industry?
  EDIT: Original comment was something like "My workplace is 80% men, so it wouldn't be feasible to put a woman in a higher position"
  Like.... dude..... come on........ come ooooon....
  
  My industry is 80% men and i've been lead tech in multiple workplaces
- thanks for your comment, but unfortunately this community is for women only - hope you understand ❤️