the Pakedge seems to be normally manageable through the GUI.
The FGT will not be able to manage the device, as FGT will be a wireless controller only for the FortiAPs. So the FGT and the other device, be it your laptop, router, raspberry PI... will be simply considered as some host or another router.
However, I found a manual on the device on an external site:
It details that you can access the Pakedge WAP-W3Gs interface via IP 192.168.1.250.
So you will need one interface, say internal3, on FGT that connects to the WAP-W3G, maybe set an IP 192.168.1.1. On your PC, you maybe have another IP as 192.168.0.1 on internal1 (just an example). Then you create a firewall policy from internal1 to internal3. Another probably from internal3 to WAN as the WAP-W3G may need internet connectivity.
You should now be able to connect to the 192.168.1.250, provided that is currently its IP address.
You can then configure the Wi-Fi AP and clients connecting to it would go online through the APs that routes traffic to the FGT which in turn routes traffic to wherever, likely internet.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.