Could grinding for loot in games be considered a form of gambling?
Well, it depends on what you are trying to prove.
You can’t go broke grinding for loot. Unlike paying for lootboxes – that stuff is outright gambling.
RPG loot does involve an element of random chance that provokes unhealthy behavior in people. Like grinding. But unlike classic gambling, you lose nothing but time doing it.
Some games do manage to add a gambling element into grinding for loot. For example, Genshin Impact has an anti-grinding mechanic that limits how many times you can get loot from grindable places in a day. You can exceed that limit for premium currency. I doubt people actually do that.