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Billings is a cool tool to monitor your hours if you have an hourly project. Plus it has some nice invoicing features. To be honest, I felt like this app was a lot more complicated than it needed to be and very overpriced. I used Quickbooks at the time and so I could have used a stopwatch and gotten the essence of what Billings provides.
You basically need to get a print out of what's in Billings and enter it into Gnucash.
I haven't worked an hourly project for a while, but when I do I'll add in what I use for time tracking.