While it’s true that the green channel is usually closest to the Lumininance channel of YUV encoding, if you really want just the Grayscale information, without any interference by poor colour encoding use this:
Channel Combiner
From: Luminance
To: Lightness Only
( There’s also a YUV to RGB converter in there that you can follow with a Channel Mixer to extract the Red Channel afterwards, but it limits grey values from 16-235 )
I prefer using the channel mixer, it might not ensure the most grain-free result, but it gives a lot of creative freedom.
I usually go for lots of red, because it brightens and evens out skin-tones