You take off for a BHS step-out on two feet, and the take-off for a gainer is a little different. You stand on one foot with the other behind and swing your free leg (which starts behind you) past your standing leg and into a split as you jump.
Also, when we say that you did a gainer, it means that your hips moved forwards rather than backwards at the beginning of the skill. If you sit and jump into your BHS, your hips sit backwards. When you kick with one foot, it causes your hips to move forwards before rotating around to your hands. It means that your hands should land near where your bottom foot began.