As others have pointed out, there's no such thing as a "guaranteed" investment.  Any investment that claims to be guaranteed is either lying, clueless, or an outright scam.  That being said, some gambling sites are better to invest in than others but there are risks involved.

I agree with others that the biggest risk is finding an honest site with honest owners who aren't just going to disappear with your Bitcoins.  Another huge risk is hackers and the security of the site you invest in.  The returns/payouts don't matter if the site gets hacked and the house bankroll disappears.  Whales/High Rollers are a smaller concern, any big win isn't good for your investment for the short-term, but it should still work out long term as investing in gambling sites is a long term play on the house edge.

I started investing a little with the site in my signature and so far I am happy with it, even though most of my profit has been wiped out (short term).  The minimum is only 0.01btc and you can invest or divest (end your investment) at any time.  This comes in real handy if you notice a high roller starting to bet big, you can end your investment (and lower your risk) and wait for the high roller to finish playing then invest your money again.  Takes time to manage but works well if you want to invest in gambling sites and want to minimize risk at the same time.  Another thing I've noticed is that weekends seem to be a better time to invest... more people playing means the house edge/odds are more pronounced over a shorter period of time (your chances of making a profit short-term are increased but risk of losing also goes up).

Personally I like the idea of investing with gambling sites...but ONLY if you are investing for the long term (and if you're gonna gamble, might as well pick the side with a mathematical edge).

For short term investments, I've started playing with a couple btc lending platforms... I lend out btc for a small percentage return.  Sure there are risks with this too, but when you've got btc doing nothing, getting a small percentage every couple days is a whole lot better than nothing.