Syncing Google Calendar with iPhone Alarms
All you have to do is to choose a long and loud ringtone as your calendar alert sound. Try it, it works, and doesn't require any technical manipulation. You can change the alert from the preferences (and not within calendar itself). 

You will probably have to make a ringtone yourself if the regular ones seem too harmless... A ringtone can be as long as 30 seconds. Here is how to make one: http://www.digitaltrends.com/mobile/how-to-make-ringtones-for-iphone/

You can also set a second alarm in calendar for an event. It will be hard to miss two consecutive alerts of 30 seconds!