You will receive a 1099-R, and you don't want to file until you have that. In most cases, the investment custodian has until February 1, 2017 to mail it out. It's still kind of early. If you have online access to the (closed) account, you may be able to get it there first.
The code G on your W2 does not represent your liquidation of the account. That code represents contributions into the plan. While you certainly want to type that in when entering your W2, it is unrelated to your withdrawal. A withdrawal would never be include on a W2.
You can enter your 1099-R under Federal Taxes-Wages and Income-Retirement Plans and Social Security. You're 1099-R is typed in here. The form will notate the withdrawn amount and the federal/ state taxes withheld.
