Can Gear VR on Galaxy S8 be used for Daydream apps I have an S8, but not yet bought any VR accessory; and I know that Samsung recently added Daydream VR support to the phone. From what I've read, Daydream case is far simpler with most of the sensors in the phone; while Gear VR has sensors in the headset itself.

So it appears to me that other than the controllers, Gear VR headset in hardware terms should cover everything the Daydream headset does. If I buy the 2017 Gear VR (for the controller), will I also be able to view Daydream VR titles, and how will the different controller affect the compatibility?
The Galaxy S8 is listed in the official Daydream-ready phones page, so you can definitely use a Daydream headset with your device.  

This comparative article points out that both headsets give access to separated stores, with different applications, noting that Gear VR provides overall better hardware and immersion, while Daydream might have a stronger "software" approach.

Now some users managed to get Daydream applications working on their Gear VR, providing you have a Daydream controller - or a second phone to act as one, and follow the (seemingly lengthy) steps of this tutorial:
https://www.reddit.com/r/GearVR/comments/5dxd8f/how_to_get_daydream_apps_running_on_galaxy_s7/
It seems that you however cannot use the Gear VR controller with a Daydream application.

If you are more inclined to spending money in place of your valuable time, maybe buying the two headsets could give you access to the best of both worlds, and avoid you the headaches inherent to getting unofficial use of the products to work for you.