Note this permission is almost completely undocumented, hence we need to feel our way to its real meaning. So this is in no way an "authoritative answer" – but more or less something between a "good guess" and a "deduction from other pointers".
The official description for this permission (as said, there's no further documentation) is:
Allows an application to modify the Google service map.
So now we can guess what that's supposed to mean. Let's do some "reverse engineering" for that to feel our way into it. Ryan already gave some pointers in his comment, linking to two SO questions:
Reading between the lines: Whenever an app wants to use (parts of) the Google Services Framework, it must declare the
READ_GSERVICES permission required – which is pretty much similar to requiring
GET_ACCOUNTS if you want to
USE_CREDENTIALS: the app first needs to ensure the required service is available at all, before accessing it.
This gives us an idea of what the Google Service Map must be: some kind of index for available (installed) Google Services.
Now that we discovered that, we can make a good guess what the
WRITE_GSERVICES permission might be intended for – and why it is protected by the
system protection level (meaning, it's only granted to "system apps" – i.e. those integrated into the ROM, which is being installed on the
/system partition): If
READ_GSERVICES is intended to figure out which Google services are available,
WRITE_GSERVICES must be its counter-part to update that service map. Which is e.g., whenever a new service was installed (or removed), to update the "index of Google Services available on the device" – the "Google Service Map".
Just one thing I wonder in this context is, why this permission belongs to the
ACCOUNTS group …