My child has been in gsis primary for 5 years since kindergarten, she is most happy and finds a good fit there, making great friends, loves her teachers and schools. We don’t feel much pressure at all. Kids are well behaved, all rounded , independent, confident and happy. That’s why they learn well.
Yes they are highly selective and putting a group of high ability kids together to learn, so they won’t get bored and always find their own type. If you fit there it will make the best of you.
We came from a non selective IS before, my kid thrives better in gsis. But if you need a lot of tutorials to keep up even in primary, then I think the school might not suit you.