Gero is counting on the fact that #19 will absorb enough energy to win. It's solidified when, later in the battle when Goku is whooping #19's ass, Gero isn't freaking out that his prediction was wrong, he's freaking out that #19 won't be able to absorb any energy. Whatever level Goku was on, it was low enough in Gero's mind for #19 to overcome.