What is the effect of introducing the main character, Dorian Gray, in this manner—that we encounter him first as a topic of conversation and only meet him later?

Introducing Dorian Gray through the conversation of Basil and Lord Henry gives us a look at the protagonist from these two characters' perspectives. We can distance ourselves from him and see him through someone else's eyes. We learn about his reputation and opinions about him—not only those of Basil and Lord Henry but also those of their acquaintances. It builds up our curiosity about the protagonist and creates suspense. We learn that Dorian Gray is an extremely beautiful gentleman who plays piano or violin. Basil is obsessed with him, likes spending time with him, and talks to him about many things. Dorian is a new inspiration for him. When we finally meet the protagonist, we can compare the description introduced by the two men with reality and see what he is really like.
http://www.gutenberg.org/files/174/174-h/174-h.htm

https://www.britannica.com/topic/The-Picture-of-Dorian-Gray-novel-by-Wilde