It really varies by subject and school.
Reading is easier to meet GD in, I have found, when looking at my own children (one was much better than the other but both GD); their school's demographic was mainly 'comfortable middle class' and about 60-70% attained GD for reading (I worked there too, so knew the data).
It's harder to get GD for Maths as not only do you need to get things right, but you also have to show skill and efficiency in calculations (although to be fair, you have to show this for EXS at KS1) - things like working out 39 x 8 by doing 40 x 8 - 8, and applying mathematical knowledge to reasoning questions. I'd say in their classes about 15-30% would make GDS. My DS was marked EXS at KS1, despite getting all but three questions wrong in his SATS, as he didn't show that kind of flexible maths brain in class.
GD in writing is the hardest to attain, as you need to be technically proficient (so virtually all punctuation and grammar accurate, and almost all spellings on the lists up to Y6 correct; these include many words regularly misspelled by adults), but also have expressive writing that also utilises things like pronouns and synonyms to ensure there's flow and cohesion. I'd say if properly assessed, 5-10% of a class like in my kids' school would attain this.
If your class demographic includes more children with barriers to learning, the figures will be lower or much lower. Maybe say, 15-20%, 10-15%, 2-5%.