Both Uses of

subtle

in

The Great Gatsby

But I can still read the gray names, and they will give you a better impression than my generalities of those who accepted Gatsby's hospitality and paid him the subtle tribute of knowing nothing whatever about him.
p. 61.8

subtle = not obvious, but understandable by someone with relevant knowledge

I see now that this has been a story of the West, after all — Tom and Gatsby, Daisy and Jordan and I, were all Westerners, and perhaps we possessed some deficiency in common which made us subtly unadaptable to Eastern life.
p. 176.4 *

subtly = in a non-obvious manner

Definitions:

(1) (subtle as in: a subtle poison)
working in an indirect or hidden way
(2) (subtle as in: a subtle shade of blue)
understated so as not to draw excess attention
(3) (subtle as in: a subtle difference or thinker)
not obvious, but understandable by someone with adequate sensitivity and relevant knowledge (perhaps depending upon fine distinctions)

or:

capable of understanding things that require sensitivity and relevant knowledge (perhaps understanding fine distinctions)