Things have moved on. It was a very progressive piece of legislation at its time. It's not so much now, very few trans people use it. What it requires is you have to have lived in your acquired gender for 2 years, which again is quite difficult to prove. So they go on stuff like when you changed your name, whether you're in work, to prove that you're living appropriately. Juliet Jakes writes about this. It was quite common, I think it's less so now, to see trans women working in charity shops, because if you've got a volunteer job, that would fulfil your requirements. You have to show payslips, or things like payslips, that show consistently use of mrs or miss or ms. So you have to show life evidence, thenyou also have to have two medical reports. And you have to answer about surgery.
From what I understand, being able to hold down a job in the acquired gender used to be a condition for surgery. It's not required for a GRC. Surgery isn't required either. As Shon says, you just have to 'answer about surgery'.
GIRES say If you have not had any surgery then the report must explain clearly as to the reason why. This could be because you are currently still waiting for surgery on a waiting list, or you aren’t medically able to or for any other reasons, but an explanation has to be included as to why this is so.
But yeah, payslips, or things like payslips, that show consistently use of mrs or miss or ms.
Or passport, drivers licence, bank statements, utility bills, letters from employer or college etc. It's recommended you send 5 or 6 docs to cover the 2 years. The horror!
I hope they don't go on consistent use of mrs, miss or ms though. That would be unfair. I can't get companies to consistently use ms no matter how many times I tell them. From everything I've read, the panel want to see docs in the new name, not title. Although I suppose they might be a bit 
if everything said mr.