That's correct. The gCode generated by Simplify3D will not do a lot of useful things on a CNC. It is the same code language, but 3D slicing programs output different kinds of tool paths because of how 3D printing is typically done. The commands are the same between 3D printers and CNC machines, for the most part, when using gCode. However, there are particular commands you would only use on a CNC machine (spindle speed, tool change, etc.).

The CNC equivalant of S3D is one of a number of CAM (computer aided manufacturing) programs. Essentially, CAM = Slicer, when comparing CNC and 3D printing. You import whatever object you want into the CAM program and it decides how to move the CNC machine to give you your final part. Fusion 360 is a great CAD + CAM software package that you can get for free. I personally use a CAM plugin for Solidworks called HSMWorks, which I enjoy a lot. You can define your stock material size and can even use other parts as your stock. That allows you to machine a new piece from an existing part (for revisions to parts, or just recycling unused parts).

There are two general types of programming for CNC machines: conversational and manual. Manual programming is entering each line of gCode by hand, which may be fun to learn but is generally not needed. Conversational programming is just using CAM software to have it generate the gCode for you, like S3D does for your prints. You'll be able to define your stock size, tool size, how fast you want to cut, what kinds of tool paths to take, and a lot of other cool things.