here is some advice from enda scahills banjo tutor , this is what he likes to use for gdae
e string 11mm or12mm plain steel, a string 16mm or 17 mm wound, d string 25mm or 26 mm wound, g string 34 to 40mm wound, these ae guitar strings he prefers them to banjo strings, he reckons that for his taste a plain a steel string is too twangy, he also says that the gauge of the g string will depend on the banjo itself, that a short scale will probably need a heavier gauge g string a 40mm, just another opinion.