Gambling is the fastest way to double your bitcoins in a day or even in an hour if you are really lucky. Life is a gamble anyways so it's up to you to decide on that.
Even I agree that life is a gamble but I do not want to risk my entire life and wealth with the gambling. If you believe gambling is based on luck, then luck will not favor you all the time. Even life is a gamble we can secure it by taking calculated risk which is not at all possible with gambling.
Yes, gambling is not within our control to make it useful to double our bitcoins by applying some strategies or with some calculated risk. Purely gambling is based on our luck and if you are lucky you may get some chances to double your bitcoins.