I've heard about the bottle of water trick before and apparently it does work although I haven't tried it myself. I've also heard something about a bit of hosepipe left on the lawn looking like a snake!!!!! I've tried the gel that is sold in pet shops - I got the strongest version to make sure and it did work; it is also not unkind to the cats. I bought my mother a battery operated device that made a high pitched sound only heard by dogs, cats, etc. although when my son visited, it had to be switched off because he could hear it!
The frustrating thing for me is that when my son was back living with me with his litter trained cat she used to come running back into the house to use the litter tray!