CoW is a strategy where you don't actually copy memory until you write to it. So, when the 10GB process spawns a child process, that child process also has 10GB of virtual memory, but both processes are backed by the same pages. It's only when one of them writes to a page that a copy happens. When you fork+exec you never actually touch most of those pages, so you never actually pay for them.
(Obviously, that's the super-simplified version, and I don't fully understand the subtleties involved, but that's exactly what GP means: it's harder to analyse)
To make it slightly more complicated: you don't pay for the 10 GB directly, but you still pay for setting up the metadata, and that scales with the amount of virtual memory used.
(Obviously, that's the super-simplified version, and I don't fully understand the subtleties involved, but that's exactly what GP means: it's harder to analyse)