Jim,
Protel doesn't support the full Gerber standard for imports (nor
exports), this is probably your problem. I believe that somewhere in the
Altium technical knowledge base you will find the details on successfully
converting most Gerber into a format compatible with Protel. It is an issue
of Protel only supporting a subset of the full Gerber standard, thus they
will not import some Gerber or sometimes just some problematic statements.
They will import what they export but not necessarily what some other
program has generated in Gerber.
Of course if you are dealing with separate aperture files (non 274X
Gerber format), you must also get the apertures imported correctly before
loading the actual Gerber layer files.