How does the GDE work- do you need a maths background, and where can it get you?

The GDE: what is it?

I am doing a political theory masters at Oxford (philosophy degree, Durham previously)
As far as I am aware, the GDE (from birkbeck) is similar to the ADE (at Cambridge), in that it gives ppl the necessary background to do an econ masters; and is equivalent to an undergrad…
I have questions: as far as I am aware, the ADE requires a numerate background (like a STEM degree)…. But I have heard no such thing for the GDE, in fact, I have heard of people with no quantitive background entering that qualification: and heard of people with degrees in things like history or politics completing the GDE…

Now:
I fail to understand how, if this is viewed the same as a degree in economics- such as, can it lead one to study at Oxbridge, Durham, LSE, Edinburgh, or UCL for their masters? If so: what is the point of doing an economics degree, when you can do a degree you love/find easy, then switch, regardless of quantitive background?! This cannot be right, surely? How can a one year course (though extremely intensive I’m sure) demonstrate the ability that a full undergraduate degree does?
So this must be incorrect?

Basically my questions:
Do you need a background in mathematics to access the GDE, such as myself for instance; with my most recent maths qualification being my maths a level?

Next: does it actually lead anywhere?
I know American universities care more about the classes (modules) you took, that would undoubtedly be present in this diploma, then I’m sure the masters could carry you to a PHD, but it must be wrong that I, with minimal mathematical qualification at an undergraduate level, be able to do this degree, and despite my good academic history, be able to an economic masters at a top school; then a PhD at another top school.
(That said, I have no real intention of economics- diplomacy or special advisor is my main love for a job, though obvs these are both very competitive so I’ll probably just do the law conversion… I was just really confused as to how this GDE thing worked, and if it was actually respected?