Getting money into gdax and Coinbase can be a hassle. If you use a bank account the deposit can take days. If you use a credit card instant buy on Coinbase the fees are high.
So for a large sum of money, and large can be different from person to person, use a wire transfer. Your money can then be in the same day if sent before cut off times.
First log into gdax.com.
Click the Deposit button.
Make sure the Bank Wire tab is selected at the top. Then copy down all the info to send the wire to. Make sure to include the Recipient Instructions/Message. This is very important. I did not include that the first time and it slowed my wire transfer down by a few days.
The wire transfer can save you a lot of time and money in fees. There is usually a fee with your bank but sometimes that can be waived. Gdax has a $10 fee as well. If the amount of money you are purchasing with is large the fees are usually less this way.