Can every DCFG be converted to DGNF?
If by deterministic GNF you mean as a deterministic grammar that is also GNF then yes, here is the paper. Normal forms of deterministic grammars 


  It is shown that every strict deterministic language may be given a
  strict deterministic grammar which is also in Greibach normal form.


EDIT
Here is a paper outlining the technique to do the conversion Strict deterministic grammars and Greibach normal form