gam·ble
ˈɡambəl/
verb
1.
play games of chance for money; bet.
"she was fond of gambling on cards and horses"
synonyms: bet, place/lay a bet on something, stake money on something, back the horses, game; informalplay the ponies
"he started to gamble more often"
2.
take risky action in the hope of a desired result.
"the British could only gamble that something would turn up"
synonyms: take a chance, take a risk; More
noun
1.
an act of gambling; an enterprise undertaken or attempted with a risk of loss and a chance of profit or success.
synonyms: bet, wager, speculation; More
Basically investing on HYIP falls under the number two category of the verb meaning of gamble. You invest in HYIP so you can get 2x 3x your investment in just a matter of short period of time. The problem with this kind of method is that those who joined in late will have a huge potential to be scammed.
I'd argue that in general there is a huge potential to be scammed, so in a way it is gambling, as you're just as or maybe even more likely to lose than to win. Just saying I don't have much experience myself with investing in HYIPs, but that's the impression I got from researching the matter.