That would be expensive since it would need R&D fund to make sure the gun worked well on the system and Forever Entertainment is a software developer, not controller maker
For the Switch it might not be that expensive, it could just be a modified version of the
Joy-Con gun, a plastic shell shaped like the Virtua Gun where the Joy-cons can fit in.
Either that or a third-party manufacturer to make the Virtua Gun for the Switch.
Virtua Gun for the PC, that would be a little more tricky. I know there is
AimTrak light guns by
Ultramarc so F3 or R&D may collab with them to make exclusive Virtua Guns for PC, I dunno.
I suppose the last sollution outside of F3 or promotional would be to DIY your own Virtua Gun. Either 3D-print the shell or take apart on old Virtua Gun and put in the components yourself.