Pituitary giants - is the fusing of growth plates dependent on amount of growth hormone in blood?
Gigantism is caused by too much growth hormone (GH) and causes increased growth in kids (healthline website). After adulthood, increased GH leads to acromegaly which leads to continuing growth of fingers, feet, forehead, jaw, nose, and lips (UCLA Health website). Note that structures like the nose and ears also keep on growing in healthy folks.

Hence, GH does not prevent closure of the growth plates, which happens normally as far as I am aware. 

Taking GH during childhood will likely lead to gigantism, and to acromegaly in healthy individuals. 

EDIT: In answer to the QUESTION EDIT: the comment by @Raoul is interesting here, as the first guy Bernard Coyne was an eunich. Perhaps that was why his plates never fused as he will have had no testosterone production. I couldn't find specifics in the other 3 cases, but my guess is they are also exceptional cases (otherwise they wouldn't have made it in the record books). As to the question title I dare say that GH is not involved in plate fusion.