Umamusume is a gotcha game

Small nitpick, but it’s actually “gatcha” or “gasha” because it comes from the Japanese word “gatchapon/gashapon”. The word is derived from two different Japanese onomatopoeias:
Gasha - The sound of a toy capsule dispenser handle being cranked/turned Pon - The sound of a toy capsule landing in the output slot of the machine.

Basically, you know those little coin-operated toy capsule dispensers that you can find in arcades? The ones that have little toys, stickers, candy, etc. inside? They usually look something like this:
Image
Yeah, these things are wildly popular in Japan. They’re colloquially referred to as “gatchapon”. There are massive stores full of these gatchapon machines. Brands will do promos for new anime, TV shows, band album releases, etc… Collectors spend a lot of money to get the rare collectibles from these machines, because not all the toys are the same rarity.

And a gatchapon game is the same basic concept, but in a digital format. You get pulls/draws/{whatever the game calls them} via some method (usually purchasing them, because that is usually how the game makes money), and then those pulls are used to get new things. Sometimes characters, sometimes equipment, sometimes new outfits, etc… It’s literally gambling, because the best stuff is virtually always gated behind some hilariously small jackpot odds.