Not sure, but have you tried:

\definefallbackfamily [mainface] [serif] [Gentium Plus]
[range=greekandcoptic]

The wiki has a page on fallback fonts:

https://wiki.contextgarden.net/Command/definefontfallback
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.

https://fontdrop.info/

Good luck!