I pretty much answered this question 2 days ago in your other thread:
https://rog.asus.com/forum/showthread.php?91492-Good-time-to-buy-ASUS-ROG-G20CB&p=636431&viewfull=1#...The 1080 is the most powerful card available that will work in a G20. Not enough power for a Ti and you can't add the needed power without extensive warranty-voiding modification. You would be better off buying a different PC pre-built with a 1080 Ti.
Perhaps next generation you could upgrade to a "GTX 1180", as long as the power requirements are similar to the 1080.
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