DDs teacher has now called me and explained to me that to get GDS in Writing the child needs to get a "way above expectation 5/5" in all the Writing bullet points in the school report. My DD got the majority at "way above" (5/5) and only one bullet point at "above" (4/5) in Writing. However, this was the key one - it had to do with the way she organizes and edits her writing.
The way she explained it: even though she had a high score in SPAG (117), and all "working above" tick marks, she doesn't yet use her grammar knowledge in independent writing. Mind boggles :)
She's also said that 1 in 5 in our school got GDS in Writing, and nationally it's more like 1 in 10.
And she's also mentioned that there is a discrepancy between teacher assessment for Writing - which she says it's way too tough, and SATs scores for Reading and SPAG.
Apparently too many children now get scores >110 for Reading and SPAG but they get EXS, not GDS in Writing, and many teachers think that there should be more alignment with these two, as they correlate.
As a consequence, the percentage of children being assessed as "Higher Standard" in combined Reading, Maths, and Writing is quite low (10% nationally), because - to be banded in this group - you need >110 in both Reading and Maths, but you also need GDS in Writing, which not many get. So basically Writing has too much weight on the combined score, it's impossible to be banded as HS if you don't get it. SPAG scores get ignored for this purpose of banding.
The other thing that I learned was that - for Progress measures calculations only - GDS in Writing corresponds to a scaled score of 113, EXS is 103, and WTS is 91. But, these scores are not communicated to parents for Writing because they are artificially assigned and everyone in the EXS band gets 103, for instance - there is no gradation.
This is all very technical and only applies to KS2, not KS1 - but perhaps it will benefit someone. :)