The ewerk is a very pricey solution to a problem that really does not exist anymore. It was designed to output various voltages at various amperage to be able to adapt to all devices. This problem has been minimized by the ubiquitous USB cable, if your devices can be charged by USB you only require 5vdc at 500 mAh for them ( iPad excepted).
A quality rechargable NiMH AA Battery is about 2000-2200 mAh in capacity. The 6000 mAh powerpack will be about 3x better than 4AA's . For reference 2AA batteries will charge a iPod Touch to about 70%. I do not know how much the GoPro or Garmin use/store so I can't make a guess as to how often you would need to charge.
Simple way to compare different battery storage is:
AA @ 2000mAh (2a) x 1.2v = 2.4
4 AA = 2.4 x 4 = 9.6
PowerPack @6000mAh (6a) x 5v = 30.0
30/9.6 = 3.12 times more "work"
The links are to the most relevant pages of my blog where I have addressed exactly how I handle charging and device management on the road, there are other pages of the blog that have additional pictures and info but these 2 are the basics. http://joesride.wordpress.com/2011/11/09/electrical-chores-today-2/http://joesride.wordpress.com/2012/01/20/final-electrical-installation-part-2/
I run a SON generator hub , the 6v Ac it outputs is converted to DC through a bridge rectifier and charges 4 (completely dead) AA NiMH batteries in exactly the time/speed you quoted, 10-12mph x 5-7 hrs. If the Power Pack does 3x more work than the AA's it stands to reason that it will take 3x as much to fully recharge it.
The electronic boxes you see on my bike will work with any 6v bottle generator as well.
I could explain this for another 10 paragraphs but I will spare you ... If you have any questions after you see what I did please just ask, I'd be happy to expound on any topic you like. If you want specific directions on how to make the Gen to USB box I have a parts list, or I could make one for you ,free assembly for ACA members
they have about 40 - 45 $ in parts in em' .
NOTE: I'm currently on a Anacortes to Key West to Boston tour so I may not reply immediately based on WiFi access.