It's really not something you can control at all. Everything that makes an application generally strong makes an applicant more likely to receive merit aid. The best way to give yourself a shot is just to apply to a lot of schools. However, if a school is not concerned they might lose you they won't pay to get you, so in that sense GPA does matter, and if you're a below average applicant for a school it's very unlikely they'll pay to get you.
This is why it can be strategic to indicate to a school in your target range that you're getting a lot of interviews, and considering schools that are better than them, as long as it's truthful. However this is obviously a risky strategy and if done incorrectly it will turn that school off your application entirely.