So sorry this happened, Rowantree! It sounds like it was truly the "perfect gift" for GD and it's unfortunate that DD2 doesn't seem to appreciate that! Please don't beat yourself up about it - it was a mistake and I'm sure you won't repeat it.
As for asking first about a gift for grands, I generally do, especially where large items are concerned. It's not that hard to ask the mother/parents. You (general GP) shouldn't "have to," but it can save a lot of heartache. Yes, it's bad manners to complain about a gift, as another poster said, but if you ask first, you avoid that issue.
I know you must have been really eager to give GD this tool bench and didn't want to be told no. But you've been told no, anyhow, and now there's tension between you and DD2 because of it. So better to ask first, I think, even if it means you can't get GD what you would like to.
Do I think DD2 should have flown off the handle like that? No, and again, it's bad manners, after all. But if she has talked to you about the size of gifts before or accused you of getting "too many," then I can see how this could have been a "final straw," even though it's such a lovely gift. In the future, please hold back. And if you really want to buy something, then ask - yeah, even if it's a book.
In the end, I think the idea of keeping the bench at your house is the best idea. Give DD2 some time to cool down and I'm sure all will be ok. (((Hugs)))