GPU passthrough requires a hardware video output for the discrete GPU. You can go looking through the moaning of Oculus Rift users[1] as they prowl laptops that do this, but it's pretty much unheard of in current standard laptops.

[1] Rift needs better video latency than you get with routing the DGPU output through the IGPU. Apparently there are now some specialized 17" gaming laptops that support a "direct mode" that might allow GPU passthrough to HDMI given software support.