How can GDAX pay no fees while transfering Bitcoin between wallets? How is that possible that using GDAX you don't pay network fees (than can go up to 30 USD) to move bitcoins between wallets?

How can GDAX pay all these transactions fee for you?
GDAX is a centralized exchange, they don't actually transfer the Bitcoins between wallets. They have a database that associates every account with how much Bitcoin they are worth.This is why transferring from Coinbase to GDAX is free, because it is the same company running both platforms.