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.
There is online the reference Gerber viewer from UCAMCO, the inventors of the Gerber format. There you don't need to install anything and you can directly upload and check your Gerber and Excellon data: UCAMCO online Gerber viewer
If you want to check your Gerbers by a Gerber viewer, have a look at the free tool GerberLogix, a product by EasyLogix
GerberLogix is an EasyLogix product 
The Gerber viewer GC-Prevue is not free anymore.