We all call him Jen-Tree, and the workers and fellow yoyo’ers at Bird in Hand call him Jen-Tree.
BTW, he’s a great kid and has great patience with the young yoyo’ers.
I’m sorry that I can’t find a video of Gentry saying his own name but take a look at this video (this video is quite clear.)
The announcer quite clearly pronounces a J (like in generation). Also show your brother the posts and tell him that everyone pronounces Gentry’s name like a J as well and even if one announcer pronounces his name wrong, it’s not quite possible that all the announcers can get it wrong so it proves that Gentry Stein is pronounced like a J.