Display Ghostscript (DGS) is functionally upward-compatible with Adobe Display PostScript, but it has been written independently. The Display Ghostscript System provides a device-independent imaging model for displaying information on a screen. The imaging model uses the PostScript language which has powerful graphics capabilities and frees the programmer from display-specific details like screen resolution and color issues. DGS is an integral part of the GNUstep Development Environment.
ViewPDF is an application to view and navigate in PDF documents. It is the successor of ViewIt, which was intended to be a universal preview application for GNUstep. Now, there is Preview.app that plays the role of a preview application, and ViewPDF is focused only on PDF documents. It uses PDFKit to render PDF content.