Converting CFG from GNF to CNF
You can just apply the usual procedure that converts any context-free grammar (CFG) to Chomsky normal form (CNF), since a CFG in Greibach normal form (GNF) is still a CFG. 

In the case of GNF, the task is, in fact, easier. All you need is to apply the following two rules repeatedly, as described by Wikipedia, until there is no nonsolitary terminals nor more than 2 nonterminals on the right-hand sides of the production rules.


Eliminate rules with nonsolitary terminals. In the case of GNF, this is used to eliminate the first terminals in the production rules.
Eliminate right-hand sides with more than 2 nonterminals.