read ( use a text editor) gsimple.cfg also fanuc.cfg (or what ever machine you are using) and of course the gsimple manual.

Gsimple works although it is picky in that tool in tool changer and material speeds and feeds must be told to Gsimple or it will just say something like no tool in tool changer. Personally if tool not correct diameter even if only 0.001mm off and tool did not have any feed and speed data for material it maybe should just refer you to a help screen explaining it. On the other hand the software author who does it free has been known to answer email as he does it as a hobby. Gsimple does not do what mastercam does but gsimple's manual isn't like the 1000 page mastercam manual is either. Gsimple manual i believe is much less than 100 pages

Ace converts dxf too
http://www.dakeng.com/ace.html