@mseder The "GSX Handling" option on the Aircraft page's configuration menu will control if the boarding simulation integrates with the GSX boarding logic. If you have GSX installed, enable the "GSX Handling" option and you will be able to control boarding through the GSX menu.
If you wish to use SimBrief and GSX with the 146, I would recommend the following method:
Generate a flight plan through SimBrief
Load into the 146
On the EFB open the OFP page and ensure the SimBrief flight plan is properly imported
Open the Aircraft page and when prompted, import the weights from the SimBrief flight plan. The weights will then be flashing and they can be edited either by clicking the left/right arrows, or by clicking on the fields and typing with the on-screen keyboard. Note, if you are editing the cargo fields, the inputted value needs to be four digits, so 450KG will need to be entered as "0450".
Ensure "GSX Handling" is enabled on the Aircraft page's configuration menu.
Open the GSX menu, and start boarding.
Mark - Just Flight