I'm working on a QB migration myself right now.
MBS offers a service for migrating QB data to GP. It ususally works well. Be advised that they will only migrate master records, customers, vendors, etc. and only open invoices.
MBS will not migrate data from the enterprise versions QB. If you want to use integration manager you can export your master records out of QB into spreadsheets and move the data in. There is also a QB ODBC driver lurking out there that will allow you to connect into the database directly so that you can link to the tables using Access or Excel.
I hope this helps