lose or profit is depends on the players getting win or lose
of course invest in gambling site is so risky
if whales come and they win big you will get lose with your investement your money will decreased
While this is somewhat true, I wouldn't say that it is "so risky". While whales might be able to take all your money, eventually you will gain it all back due to the house edge. The main worry is not whales, but exploits in the site that let people take money without any consequence.
As long as you choose a secure, well trusted site it should be fine.
How much money do you want to invest?
While I mainly agree with you here, investing with larger amounts of bitcoin is usually a good way of investing into such sites- I have had experience with amounts of less than 0.01 BTC disappearing in less than a week due to whales(don't invest tiny amounts). It's s not completely foolproof, and it's relatively slow profit, but by no means should you invest all of your bitcoin into a casino.