It has been shown that if P != NP, then GI is in that valley.
"In computational complexity, problems that are in the complexity class NP but are neither in the class P nor NP-complete are called NP-intermediate"
"Under the assumption that P ≠ NP, Ladner explicitly constructs a problem in NPI, however this problem is artificial and otherwise uninteresting. It is an open question whether any "natural" problem has the same property."
Could you point to a reference? Is this true even if P != NP, but NP = coNP?