It defaults to the XY plane but I need to use it in the XZ plane.
You can adjust the plane with the dialogue that pops up while spinning (you can change on the fly as long as the dialogue does not close), but I find it easier to set the view to the desired plane and use the 3d cursor as pivot.
Here I set the view to the side (numpad 3) and used the cursor as pivot. It has spun as I wanted, note that in the dialogue the axis says X 1 in the axis section if you set Y to 1 as well it will also spin on the y axis.
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I did this also but it always spins around the Z axis when I use the gizmo.