Whaling

[Dave Paprocki]

Pertaining to, or employed in, the pursuit of whales; as,
a whaling voyage; a whaling vessel.

[Renato]

The practice or industry of hunting and killing whales for their oil, meat, or whalebone.

[Tanya Shivari]

To whale in a game or whaling is a term for gamers or gambling addicts who spend a good huge amount of money on video games or casinos. Since a whale is huge and they pay hugely for gambling or gaming, you can see why [this is a thing].
With gamers, microtransactions existed for them within free-to play games (e.g. [loot boxes], in-game currency, booster packs, cosmetics, etc.), gacha-like games, mobile games, battle-royale games or anything along those lines.
Gamblers are people who whale tons of cash on [slot machines], poker and other natures of casino games and gambling.
Whales was a phrase first used by video game publishers to describe their main customers who insufferably liked to gamble their money for in-game items.

[Julene]

the activity of hunting whales