Changing GPU

There's no way to manually control this.  Whether it happens automatically depends on how the display outputs are wired internally, but if you have a G-Sync system, you're probably out of luck for the reasons I'm about to describe.  In most systems with dual GPUs, all outputs are wired directly to the Intel GPU, and the discrete GPU runs only as needed.  When it does, it works as a render-only device, passing completed video frames to the Intel GPU for output to the display, since again the displays are all connected to the Intel GPU.  The advantage to this design is battery life, because the discrete GPU can be fully disabled when it's not needed.  The downside to this design is that there are certain features that require support from the GPU physically managing the display output, and some of those features are supported by NVIDIA GPUs but not Intel GPUs (at least as of this writing).  Some of those features are 5K resolution, HDR, VR, stereo 3D, and....G-Sync.  That's why in some systems, particularly gaming-oriented systems, at least some of the display outputs will be wired to the NVIDIA GPU instead so that at least some of those features can be used.  The downside is that if you have a display attached to any of those outputs, then the discrete GPU has to be active to send a signal to that display, even if there's nothing graphics-intensive going on.  But for gaming systems, Dell is figuring that the target market will care more about features than battery life.  On some of these systems, only the external display connectors (and in some cases only some of them...) are wired to the discrete GPU, while the built-in display is always driven by the Intel GPU, in which case the discrete GPU would be disabled when only using the built-in panel.  But if your internal display supports G-Sync, it would have to be wired to the discrete GPU.

The only way to have your cake and eat it too would be if Dell employed an even more complex design that to my knowledge only exists on the Precision 7000 Series models.  They have DisplayPort multiplexers on the motherboard.  All display outputs are wired to the multiplexers, and then the multiplexers are wired to both the integrated and discrete GPUs, and then the user can toggle a BIOS setting to decide which GPU controls which outputs -- but even there you wouldn't be able to switch which GPU controls which display on the fly.