Here's the main problem: People dont live in their Google Calendar.
Your main value prop is: Know what you have to do in the next 48-hours. But the problem is, I have to go open up my calendar to see that with your app...why even install then, when GCal can tell me what I have up in the next 48h.
The key would be for it to run in where I spend most of my day -- my inbox, not my calendar.