error message re: "gs"

[BajaRobin]

I work on an IMac - latest version Os X 10.9.4 - just found the XnView on Gizmos and downloaded it - all good looks like just what I have wanted. - My question is, what is error message: To open "gs" you need to install X11. - I don't understand what it is referring to, as it seems I can see all my pics??

Also is there a work flow suggestion, or manual for this program??

[xnview]

gs is used to view pdf/ps files but no need to install X11