I'm in the middle of writing a blog post about this and it's called: The paradox of the LoA Vs Reverse Psychology.
To answer your question, gratitude is not the only manifesting force there is, it is however, the most pleasurable and pleasant one. The simple act of thinking with feeling, regardless of what that feeling is, causes attraction. Even strong thoughts of anger, resentment, complaining etc, have an extremely powerful gravitational pull just like gratitude but they manifest different things. What we don't see about these people who win the lottery without ever expecting it is their thought process. Maybe, through all this complaining, they have strong images in their mind of winning the lottery and those images along with the strong emotions attract the experience of winning the lottery.
