Lootboxes.
Players have a random chance of getting crate while playing the game. Each crate is a pool of item cosmetics with various levels of rarity. To acquire one of them the player must purchase a one-use key with real money. Expending the key on a crate initiates a die roll that determines which cosmetic is unlocked.
That's the gambling they're responsible for. What gambling players may of afterward is not the same conversation.
Signal.
Wired had an interview with Signal's President last year that I found enlightening and provided an entry point for me to self educate further. Here's an archive.org snapshot of it: https://web.archive.org/web/20240828100224/https://www.wired.com/story/meredith-whittaker-signal/
For the click-averse here's an excerpt I find compelling: