Not sure, but have you tried:
\definefallbackfamily [mainface] [serif] [Gentium Plus] [range=greekandcoptic]
The wiki has a page on fallback fonts:
Searching for "greek" has numerous hits, including greekextended.
Have you verified that the Greek glyphs are in the fallback font you've selected? Sometimes I'll use a site like FontDrop! to make sure that there are, in fact, glyphs in the character ranges specified.
Good luck!