I have the latest version of PvZ 2 and have finished the first world but how do I get to the next without a key (for free)? I tried playing over and over again some of the stages and kill "Gargauntuars" but at no luck - I don't see any keys dropping.
There is no guaranteed way to progress (besides paying). The point is to keep grinding and hoping that a key will drop. Meanwhile the creators keep hoping that you'll lose your patience and buy a key before you get one for free. Note though that if you enjoy the game, you should support the creators in some way (but arguably not by reinforcing their questionable behaviour).