Can I install the stock firmware of my gear S with Kies3?
No, Kies can't be used to install the stock firmware of the Gear S device. You will ned advanced programs for this job. There is one that's being used to flash stock to almost all Samsung devices. It's a called Odin and has the capability of writing stock firmwares on Samsung phones and devices.
You can find more about the program and flashing procedure using the XDA Forums where this subject is discussed: http://forum.xda-developers.com/gear-s/development/definitive-guide-to-rooting-gear-s-t2973455