Wondering if anyone has attempted this. Someone gave me a Fortigate 300C, and I was told by that person that you can put PfSense on that box, but there isn't much out on the web for accomplishing this.
Fortinet uses their own Asics so chances are low. Fortinet > pfsense tbh. For this to even be physically possible you have to have access to the bootloader/bios. Then after that you can see if the ASIC is supported(doubtful) and is 64bit(required by pfsense now).
Fortigates certainly have a CPU aside from the asics. The 300D which replaced it had a i3-3220 (Ivy Bridge) @ 3.3Ghz so set your performance expectations accordingly. As said above even if you are able to get something else installed and booted you'll not be able to use the asics. Also, the 300C boots from flash as it has no other internal storage.