Gambling is definitely not equal as invest. Invest need you to do research and understand the system. Like what you say, you can hold on to your bitcoin to profit later on because you understand about the volatility of the price of bitcoins. However, in gambling, you just have to depend on the rng of the system to determine whether you win or lose.
i think buy bitcoin itself is include as investing for future and you don't have to play gambling. because gambling can make you loss all of your money, but when you buy bitcoin, then you make an investment for future.
What you said it correct. When you buying bitcoins means is a type of investment already but some people want to grow their bitcoins so they choose some better options to invest like lending money with proper collatral and investing on some good casinos than just gambling money. Gambling only brings lose to you and you never make profit from it.