In my course materials, there is one sentence about how if CFG is in Chomsky Normal Form, it is not regular, and if it is in Greibach Normal form, it also is not. But when a grammar is **simultaneously** both GNF and CNF, it somehow implies that the grammar is regular – why is it so?

I struggle to understand what would the productions of the grammar that is both in CNF and GNF look like. As in GNF, the production rule is in form: $ A \rightarrow aA_1A_2…A_n$ such that the nonterminal sequence can be empty, I think that the only way a grammar can be in both normal forms, is when the production rules are in form: $ A \rightarrow a$ .

Even if this is the case, why does it imply that the grammar is regular? What is the relationship between regular and context-free grammar?