I have read the whole one piece manga but i found that there is no giant having a devil fruit power. I think that
There might have been a person (ancestor of giants or first giant) who ate a special fruit and became very large and got a power to pass his power to his children so that they have the same strength and physique. Now as the giants may have got their size from devil fruit power so they cant eat another fruit.
It is just my thinking. If you have any other view then please give.
But do you think (if i m wrong) big mom would have a devil fruit power?