Does this $475 Southwest Airlines gift card have fees or an expiration date?

No. The gift card has no fees and does not expire.

How can I redeem this Southwest Airlines gift card when booking online?

After selecting your flights and entering passenger information, choose “Apply Travel Funds,” then enter the gift card number and security code (PIN) and click “Apply Funds.” If the gift card does not cover the full total, use another form of payment for the remaining balance.