WElcome Jool!
I was told the same thing by the Gifted coordinator when my son was pulled out for math with her. He's in 2nd grade. She said the first thing I tell them is that I can't give them 3rd or even 4th grade work. Instead the spent the 1hr/week for 6weeks playing chess, Blokus, some other games. Now DS had a great time and I was so happy he had this bright spot in his week. But NOTHING changed in the classroom.
We have no such animal as a GIEP. But it's only worth the paper it's written on if the school doesn't follow it. I wish you luck in your advocacy. I don't have any advice, others here are more experienced than I.
I haven't applied to DYS for my DS7 but we're still welcome here anyhow.
Dazey