Yes, sub with ginger powder.
For the amount: go by taste. You can use quite a bit.
It will not taste the same, but it will still taste good.
I've had to do it plenty times, especially cooking Indonesian food.
Years ago fresh ginger, galangal etc were hard to come by in Holland, so we used the dried (powdered) version. We even had to do this with lemon grass.
I assume you don't have fresh galangal either, otherwise you could use that.
It's more peppery, but a...