Have you even look at ACP? it's literally 4th option in Avatar settings.
Users can, and do, have different email addresses for different purposes so the email address used for a phpBB board my not be the same as the one used for Gravatar.
Well you can propose the idea for it to be changed here.
I do not have 20 but I do have around 10
As I can see, you are a developer and an advanced user, most people are not like you