I found a lead, in a sicha from toldos, 5728, section 14, note 109, it gives a references to igeres hakodesh of the Rebbe volume 23 page 57 (don't have it with me at the moment)
The language in the sicha is: "The Gra himself, in his commentary to Mishley, brings that it's forbidden for one to make a psak din in nigleh, if he doesn't know the inner dimension of Torah" (free translation of a translation (originally from Yiddish, I'm reading the Hebrew translation, and now translating from that translation to English), but you get what you pay for)
The letter mentions various references to verses in Mishley where the Gra mentioned this concept, one would have to look them all up to get a clear picture of what's happening, here are the references to Mishlei mentioned in the letter by chapter and verse:
5:18, 7:12, 4:22, "and mainly" 20:9
Then it mentions also his explanation to "Heichalos" to parshas pikudei, 17
As well as Tikunei Zohar 107:3, then it adds in bold letters "and more, in many places"