HDRI studio does require GI to light the scene, however, if you have a scene with lights in it already, it is still useful for quickly adding reflections.
I think the latest version has a button in the HDRI object controls that says 'add render settings', which turns GI on if you use it's presets. Otherwise, you need to turn that on yourself in render settings.
You'll know if it's installed wrongly because you'll get texture errors on render.
Hope that helps
CBR