Mapping Shift-space: how to tell if its possible?

I'd like to map shift-space <S-Space> to <PageDown> or something.

After a lot of googling I find several mentions that some setups are st
vim differentiates between <Space> and <S-Space> and some not.
It seems not for me since :<C-v><S-Space> outputs only a space.

I didn't find any mention what in the setup makes the difference?
I'm using gvim not console.

I tried setting gnome configuration option Keyboard Preferences->Keyboard Layout Options->Using space key to input non-breakable space character->Space key outputs non-breakable space character at second level in the hope that this would generated a different characters that vim could detect.  But if it does, it isn't <S-Space> apparently.  And that solution seems undesirable for other reasons, since I don't ever really want unbreakable spaces creeping in.

Advice?

Thanks,
Britton

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