My wife has Ghusle Janabat Wajib on her and got her periods just now (Haiz).
Can she perform Ghusle Janabat while in Haiz?

There is no effect to performing Ghusl al-Janabah when she is in Haydh for the purpose of prayer. The only effect is to remove the Karaha (negative effects) of staying on Janabah. So yes she can do this Ghusl but not with the intention that it’s wajeb to do so, but just with a general intention.