GDAX uses a maker-taker fee model. Orders that provide liquidity ("maker orders") are charged different fees than orders that take liquidity ("taker orders"). The fee is assessed as a percentage of the quote currency.
For the detailed GDAX trading fee schedule, please see https://www.gdax.com/fees.
ACH Deposit: Free
ACH Withdrawal: Free
SEPA Deposit: Free
SEPA Withdrawal: €0.15 EUR
USD Wire Deposit: $10 USD
USD Wire Withdrawal: $25 USD
There are no fees for having a GDAX account or for holding funds in your GDAX account.