Not to pick nits as it were, but why would you use Blender to process GCODE? The STL file should be loaded into whatever slicing program runs best for your 3D printer. It will generate the GCODE your printer requires. Now, that said....................It occurs to me that what you might be running into is a memory problem on your computer. STL, contrary to popular belief does NOT stand for Stereo Lithography. as in the 3d printing process. Rather, it stands for Surface Tessellation Language. Its a process whereby your model and all its faces, is chopped up into a whole bunch of little triangles. This triangulation of your model adds a LOT more geometry to your file that needs to be processed. A four sided square for example, has 4 straight lines. After an STL conversion that same square may now have been divided into two or 4 triangles, adding 1 to 4 lines to your drawing. Multiply that over a few thousand times on a complex model and you can see where the processing of such file by any 3D program, Blender included, may take a while or even crash if you don't have the computing power to handle it.
