A gift card is always worth less than cash, how much depends on various factors.
At least 5% (rewards and inconvenience) but closer to 10-15% in my experience.
For a 20% discount on stores I use regularly I’ll get the gift card (usually buy $50 get $10 free).
A $X Amazon gift card is more valuable to me than $X in cash. As in, if you gave me a choice between the two, I’d take the gift card.
If you look on the gift card market places, like Raise, you will see many gift cards are well below face value but there are also many that are rarely below 1% value like Amazon and Walmart.