What is a gacha game?

Gachas are characterized by the fundamental way some, or all, of gameplay is gated: by a lootbox-adjacent mechanic.

The difference between gachas, and say Counter-Strike 2's lootboxes is that you may play all of what Counter-Strike 2 has to offer without ever opening a box, and not in a "technically, if you grind for 234 hours, you will be able to afford [weapon] for free" kind of way. The only things that are locked behind a loot box (however expensive it is to open) in Counter Strike 2 are entirely cosmetic.

For contrast, take the quintessential "Gacha game": Geshin Impact.
In Geshin Impact, to be able to play in a certain way, such as playing as a certain character, or using a certain weapon, the only way is to open a loot box. You may not buy the character from the shop directly, the only way to get what you want is to gamble.

Sure, there are ways of gambling for free built into the game (this is a player rentention mechanic, gachas are necessarly full of them otherwise they'd be extremely frustrating), but the fundamentals don't change: in order to experience the full game, or indeed any of the game at all, you must gamble. Some would argue that the entire purpose of the game is the gambling. Dragon Collection, one of the original gachas, was actually literally just a collection game with no gameplay attached. The entire point was to gamble to get the dragon you're missing from your collection. It was quite successful for a browser game of the era.

Of course, this is a spectrum. Some games can be located somewhere in the middle, where most of the gameplay can genuinely be accessed for free, but the truly powerful elements are behind a lootbox wall.

Also note that Gacha games are not to be confused with so-called "freemium" games, where the game favors paid players much more than free-to-play ones. while the two are obviously related, the difference mostly comes down to the fact that freemium games almost always guarantee you will obtain something powerful by paying every time, while Gachas hold no such guarantees and can very well reward you with "filler" items even after paying.

In conclusion, gachas are defined not by "some amount of randomness in terms of player rewards", but large amounts of randomness in access to gameplay to begin with.

Gacha games are named after gachapon vending machines, which entice players to spend in-game money to receive a random in-game item/weapon/character. They are not only cosmetics, but can add something useful to your adventure or improve your gameplay.
These games play on collecting rewards, often inducing FOMO on players because related to time-limited events.

The access to the "pulling" mechanic is usually connected to an in-game currency/material that builds up over time, so it can be obtained for free. The idea is that you receive a chance to pull after spending many hours farming the currency doing daily and repetitive quests, or you can spend real money in order to bypass the grind.
This kind of games are designed to be addictive, but are totally playable in a free2play way, if you have patience.

Other features present in gacha games are:

• Banners: instead of one giant pool of items, there are limited-time storefronts that feature a specific reward with an increased drop rate
• Pity system: it guarantees that after a certain number of unsuccessful "pulls", the game will automatically grant you a high-rarity reward
• Power creep: the gradual introduction of new characters or weapons that are slightly stronger than the previous ones, this is meant to keep players spending on new banners