Hi there,
I see this post is frequently read here in the Forum and I would like to add a comment...
Basically Fusion Electronics has an import for Gerber data. As Edwin wrote in his initial answer, the import function is limited to basic Extended Gerber RS-274X format and mainly used to import simple templates for a PCB design.
As I understand your post, you wanted to import your Altium design files into Fusion and mill out the PCB board afterwards, right? As Edwin already wrote, there is no direct import of Altium schematics or boards into Fusion. But you could try to export in Altium the so-called ACCELL-ASCII format from your board/schematic. I am not 100% sure how this export option is called, but it is something like Pads Ascii format or the like....
If this is done, you could use an User Language Programm named import-accel.ulp and bring the design into Fusion Electronics. If this worked, please check your board thoroughly.
Now you save your design in Fusion and use the milling path to get your board milled. There is a nice video post that describes how this works in Fusion 360.
Link to Milling out a PCB in Fusion
Regards,
Richard Hammerl