Both "index funds" and "ARM" is a gamble because you're advocating investing in them without regard to price. In both cases you're just buying something based on it being popular, not because it makes a good investment.
If you do the math and you are able to calculate your returns, in advance, with some reasonable accuracy, then you're investing.
If you're just buying something without regard to price because it feels good (including index funds) or fits your emotional state- then you're gambling.
I've got no problem with speculation-- which is informed investing in high risk / high reward situations, but that takes even more analysis, not less.