Gnumeric will systematically iterate the model by changing the input value to achieve the desired result, if possible. If goal seek was successful the tool displays the message ``Goal Seeking with cell __ found a solution''.
It is possible that Gnumeric does not find a solution that generates the desired result. There may not be such a solution for the model, or, it may be too difficult to find. For example, the mathematical function behind the calculation may have many non-continuous points.