Hey there, I have the GL502VS, so I will answer with what i already know, since I was already in your position when i bought this laptop.
G-sync: I have a 60Hz screen, so it wasn't really a big difference for me. Hardly noticed it, maybe that's just me not paying enough attention though. It's about individual experiences.
TB3: I had this debate internally when I decided to get the GL502vs, and decided that GTX 1070 vs TB3, i will go with the GTX 1070.
was it the right thing to do ? well, for me yes.
TB3 upgrades after 3 years or 4 is a really long shot. There are enough rumors around about the bandwidth issues. and from what I have seen in the GTX 10xx series, you cant bet that your current laptop CPU wont throttle down, if you throw a new GPU at it 3 years from now.
So I would advise you to look for a better GPU if you want to be future proof, going only with TB3 thinking it will protect your machine from being outdated is a big bet.
USB-C ports: I haven't used it yet. They say everything will be moving to USB-C soon.
It's available in almost every new laptop now anyway, so who cares.
Hope I helped you.