The actual definition for gambling is betting your money for catching others money, and it is only the understanding of people about the gambling in their minds that some understand it as just game and some understand it as an investment.
Yes, correct putting our money at risk to catching others money is gambling. But in gambling making profit is not that much easy. So I think depositing our money to casino's sites is the actual definition of Gambling.
To me, gambling is not about the investment. It is an uneducated way of risking your money. A lot of people think they are investing in a lot of things but they are more towards gambling. Like the US crisis that leads to many people over-invest in housing just because they think they can make quick bucks where they dont even research anything about it.