If you're watching the price of bitcoin at the moment then you should neither gamble or invest atm. Just keep hold of your coins & ride this train to $5000 per bitcoin

if you are not a risk taker , then yeah better to not do both. but when you want to make your bitcoin keep growing in high risk way , go for gambling. feel not safe to do that? just put your money on altcoins or ICO . you could adjust it anyway based on your condition and needs. but everyone agree that investment always more promising compared to gambling.
Putting your money in altcoins may be just as risky except for the fact you can get out when you think you're starting to lose

But then again, if you're betting in small increments of your total bankroll it gets more and more simmilar.
nothing free risk when you are building business to grow/make money, face that fact . then it's really different when you gamble with small increments as you have said, you would get heated faster and wouldn't make good decision when placing bet, then you lost it instead get profit, where investment wouldn't work like that , you wouldn't lost with the capital unless you put it on HYIP.