Gwen Stefani, to me, is kinda like Malin Akerman. Malin stayed true to her own grungy style as seen here, here, and here. Gwen has done the same - her style is sophisticated with an edginess to it. You can see that in today's outfit - the grey tunic and black coat are classy and modern, while the black boots add the edginess that Gwen is known for.
While it is not my preferred look, it really suits her and she accessorises well with the bright red lipstick and perfectly styled hair.