When you generate Gerber files, there are two formats possible. One is RS-274X which has the apertures coded with the data. The second is RS-274D which has a separate aperture file.
It sounds like you generated the RS-274D format. If the apertures are not defined, Gerber import will not work properly. If you still have the PCB file from which the Gerbers were created, you can load that file, close it but leave PCAD running, then load the Gerber files. The aperture table used by the PCB file will already be loaded, so the Gerber files will use that table. The only other way is to manually enter the aperture file.
There is a URL that describes how to create a PCAD PCB from Gerber files by manually loading the apertures:
h**p://www.jlab.org/accel/eecad/gerber.html
(*>t above)