If your stem grades are better than your cumulative GPA you've got a decent chance of getting interviews at the Tri-I and WCM. SKI is competitive as hell. Being a Gates Scholar will help, As for your presentations, were they conference, or symposium at your school? School symposiums don't count for anything. Feel free to PM me for advice, i went through this cycle this past year with similar stats to yours and i'm going to be attending one of the programs you've mentioned.
I say interview b/c that's the hardest part. Once you've got the interview your stats become irrelevant, the focus is on you being able to talk about why you feel the program is a good fit for you, and your past work. Being able to discuss your research in depth and show that you understand it. if you've got a 4.0 but can't carry a conversation you're not getting in