Every definition of gambling I've seen includes some variation of "winning a thing of value from a game of chance", and while loot boxes for in-game rewards skirt that (the thing you're getting is typically described in the fine-print as having no monetary value), Valve's user-driven marketplace is setting a real-world value.

Put another way: EA FC's card packs or Genshin Impact's gacha spins are worth nothing outside of those games, but I've sold Counter-Strike skins to pay for a decent chunk of my Steam Deck.