For languages with a "not-equal" restriction, the easiest approach is usually to first find a grammar that corresponds to the language with an "equal" restriction instead, and then change it to require more of one of the things.
In this case we have a number of a
tokens followed by an odd number of b
tokens where the constraint is on the number of each. For the equal case, that becomes just
S → aSbb | b
a single b
with the same number of a
s and pairs of b
s wrapped around it.
To make it not-equal, we need to add either extra a
s or extra pairs of B
s, but not both:
S → AS' | S'B
S' → aS'bb | b
A → Aa | a
B → Bbb | bb