"That is not so good."
Translation:그것은 그리 안 좋아요.
My impression is
그리 = 그렇게 ( = to such extent/that ) are used as intensifiers while
별로 ( = (not) particularly, (not) so ) is a downtoner
• 그것은 그리 안 좋아요 = That is not as good as that = That is not that good (coll.)
=> 그것 is "good" but Speaker disagrees with such assessment.
• 그것은 별로 안 좋아요 = That is not so good
=> 그것 is "not good". Speaker agrees but does not want to express it bluntly.
Yes, an adverb placement can be flexible depending on which word it is meant to modify.
In the case of 그리, this adverb is used to modify either
•descriptive verbs (/adjectives) of negative meaning s.a. 어렵다, hard; 게으르다, lazy etc. => 그리 + nV
• Negated descriptive verbs/adjectives i.e. one in the form 안 V/A or V/A지 않다 => 그리 + 안 V/A or 그리 + V/A지 않다
In this example, the adjective is 좋다, good. Its negated form can be either 안 좋다 or 좋지 않다.
그리 is then applied to modify either of these negated form. Hence, 그리 (안 좋다) or 그리 (좋지 않다) = be that (not good) => be not that good i.e. be bad-ish.