I was thinking to get G-train from Wuhan Railway Station to Xian. Is it the best option?

The bullet trains depart from WH Railway Station to Xian North Railway Station. It takes around 5 hours. It will be a nice choice for you. :)
Besides, you can take the direct airport shuttle bus to the Main Railway Station. The operating hours of this bus line are from 09:20 to 21:00 at intervals of around 30 mins. The ticket fare is CNY 37 per person. It takes around 70 mins on the way.

Thank you very much! One thing. Is there a number of this bus? How can I find it?

This bus is called Qingshan Line. I’m sorry that I didn’t find the exact location of this bus. Upon arrival, you may ask the staff to lead you to find it. Nice trip.