Yes, any CFG can be transformed to an equivalent grammar in Greibach Normal Form. If the original grammar is ambiguous, so will be the transformed grammar, but the reverse is not necessarily true; the transformation can introduce ambiguity.
The above claim assumes you are using the relaxed definition of GNF, which allows the production S→ε
but only for the start symbol S
. For the strict definition of GNF, only grammars which cannot derive ε can be transformed; strict ε-elimination will result on a grammar which recognises every sentence recognised by the original grammar except ε.