The way it works is really very simple, you make your first deposit of £20, and play whatever games you like the look of during the first 48 hours of your account. If you end up busting out on your deposit, Gala will chuck you a fresh £20 in your casino account for you to have another blast. The £20 has just a 1x wagering requirement, so once you have played it through the games or slots just one time, you are free to make a withdrawal of whatever money is left in your account if you wish to.
These kinds of offers make very little money for the casinos of course, so its easy to understand why they offer them only to new players, although I do sometimes receive similar risk free offers from the better casinos with whom I deposit regularly. The only trouble is, if your willing to be tied into a wagering requirement, the 400% welcome bonus offers are probably the better bet – you could grab £80 in bonus funds for that same £20 deposit you were going to use with this offer, and whilst you are tied into a 32x wagering requirement, it’s a whole lot more fun in my experience playing at an on-line casino with a balance of £100 rather than just £20.
Still, if your really averse to losing money this is a great way of trying out one of the better on-line casinos for almost zero risk. I’d recommend reading up on the different offers available and choosing carefully before going for this one though, as good as it is.