I am not sure what you are asking, about running it off line as Mach3 does that just fine, without a machine attached to your computer.
Maybe you don't understand how to install the screen set, so here is how you do it.
First, download the screen set from my web site: http://www.mrrace.com/SimpleMill/
Click on the download link to get a copy of simple mill. Un ZIP the file and simply copy the SimpleMill folder to your Mach3 bitmap directory and copy SimpleMill 3A.set or SimpleMill 4A.set to your Mach3 directory. Select View from the Mach3 menu and click on SimpleMill 3A.set or SimpleMill 4A.set to load either screen set.
You can now load and run your GCode without any mill attached.
Please let me know if this is what you needed,