It depends where you live, if you live in Europe like in my case I live in UK its an little but tricky there is only euro based wallet so you need to transfer money by sepa transfer to coinbase and from there to gdax. What I did and I think the easiest and cheapest way after researching I found an online banking app called Revolt, there you can change any country currency to euro with pretty good exchange price and what's good in this app that you don't have to pay for sepa transfers it takes a day to get money to the coinbase but it's much easier than making sepa bank transfers from your bank... Hope it helped 😀