Turns out the correct way of saying this is:
I never knew my grandmother as she died before I was born.
Am I right that the reason past simple is used here is because my grandmother is dead and there is no way for me anymore to know her?
And if I said
I've never known my grandmother.
it would imply the existing opportunity to know her (did I use "know" correctly here or should I have used "get to know"?), i.e. that she is still alive?
Or maybe the better way to explain this is that the time period (before I was born) is already finished? If that is the case, then could I say
I've never known my grandmother
(without specifying a time frame), even though my grandmother is dead?
Maybe you know where I could find more details about the particular rule that applies here?