All the old world garcinias are officially classified as dioecious. But very few actually turn out to be truly dioecious. Many of them produce some hermaphrodicte flowers or produce fruit without pollination, as with the mangosteen. But to be on safe side would be better to have more than one tree.
The only garcinia i know that may be truely dioecious is G. praniana. And even there i'm not sure because the supposedly male flowers have an ovary, but that ovary is usually sterile. But even in this species it's possible that they may produce fruit w/o pollination, or so i've heard from some growers having solitary plants that fruited.
Oscar