When you try to push expenses to QuickBooks for the first time you get the following error -

This application has not accessed this QuickBooks company data file before. The QuickBooks
administrator must grant permission to access this file for the first time.

Open QuickBooks with the correct company file, logged in as the QuickBooks Administrator, and in single-user mode.  You must have Administrator privileges. From the top QuickBooks menu; choose Edit, Preferences, scroll to Integrated Applications,and choose the Company Preferences Tab. 

a) If the Gorilla Expense (or Reefin) program is not listed, exit the Preferences section and run the Gorilla Expense integration again. A popup will come up asking you to grant permissions to Gorilla Expense (or Reefin) to access QuickBooks company file. Click "Always Allow Access"

b) If the Gorilla Expense (or Reefin) program is listed, make sure that the top two boxes on this screen are not checked. Below these check boxes is a box listing applications that have requested access to your QuickBooks company file.