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For example, I have a monthly income from my IRL job worth $1000 and usually I have remaining $400 after I spend it for all what I need to pay (bills) so it wont be a problem for me to take a loan of $400 (better to take lower than this amount).
This is quite different from taking a loan to gamble. With this example you shared, you control some of the odds such that as long as you put in the hours, you're expected to be paid which would then flow back to cover for your loan.
With gambling, it's different. You don't control the odds since it's all a probability. You might take that loan and lose every bet you take in a bid to repay the loan.