It has GPS, it's also stated on the official website. https://www.polar.com/uk-en/ignite "Polar Ignite is a waterproof fitness watch with advanced wrist-based heart rate and integrated GPS."

Yes, when you start training session swipe down the screen and 3 buttons will appear, the far right is a bulb that means that the watch will stay on until the end of the session (more battery consumption though)

Yes, you can set which channels you receive on the clock and can read there. Replying is not possible, of course.

Yes, according to the manufacturer, this chest strap is compatible with the watch. See: https://support.polar.com/en/support/compatibility_with_my_Polar

Hello Djjojo No cannot be accepted. Griess

No, the clock does not have music control built in.