Many people said the gambling is just a game and another people said it is Investment. How do you think?

Wagering of money or something of value on the event with an uncertain outcome with the primary intent of winning money or material goods. Gambling are requires three elements such as consideration, chance and prize.
Very exact definition, so it does not mean that playing in casinos are only considered gambling because it was not specified in the definition, even trading itself is also a form of gambling because we want risk our money to get a better price which is the profit we will make.
If we want to make money we need to take risks. But the risk in gambling is much higher. However it is strange that people consider gambling to be a mode of earning and not mode of entertainment and hanging out. If you want to earn money, get a job or start your business. Learn some new things and gain some new skills. Use them to earn and not roll dice to win bets.