People think impact = sales. Gaga is (or was) successful, no one is trying to deny that. But Björk has inspired people for over two decades, has had critical acclaim, awards, some chart impact (specially in the UK, in the 90's), and many, many people in the music industry, both mainstream (including Gaga!) and indie, acknowledge her as an influence. Björk is, if anything, an icon closer to David Bowie or Lou Reed, not exactly the GP's definition of success, to be honest, but still, are huge and respected.
Gaga could be up there, in a few years, if her work was more consistent. Which I don't believe it would happen, but I would love to see that. Oh, and by the way,
yes, Björk was nominated for an Oscar, actually. Where do you think the Swan Dress is from?
Honestly, TRF.