Gerber viewer
TARGET 3001! is not a Gerber viewer! The TARGET Gerber import is able to read files containing XGerber (RS-274-X) and Excellon data from various other CAD packages with the aim of importing them layer by layer into a TARGET project for further edition.
If you want to check your Gerbers by a Gerber viewer, have a look at the free tool GerberLogix, a product by EasyLogix
Image:GerberLogix is an EasyLogix product
The Gerber viewer GC-Prevue is not free anymore.