Can I trade in my iPhone 13 pro max for a gift card?

iPhone 13 pro max is not yet on Apple's list to trade in, see this Apple article: Turn the device you have into the one you want.

https://www.apple.com/shop/trade-in

So you have to wait.

for a gift card

Yes see the same Apple article.

Not yet. You're likely to get better value by selling it than by trading it in. Especially if it's unlocked and you can provide the original receipt to the buyer.