<p>I have generated a few 3D prints in G-code using KISSlicer and Slic3r, but when I load them into Ultimaker Cura I'm unable to get anything. The number of layers says one but I cannot see anything and print option is not working. I have viewed these G-codes online and they are fine. I'm using a Lulzbot KITTAZ with v2 toolhead (hexagon) and I can print only using Ultimaker Cura. Please tell me what the reason for this is.</p>

Cura prior to version 2.5 does not take G-code as an input. I'm not sure what you mean by "print only using Cura." Don't you have a SD slot on your control board? For that matter, why can't you drive the USB port from Slic3r?
Remember: G-code is the equivalent of "compiled code," the raw commands which drive the printer, while STL or OBJ, etc., are the "source code," which you edit to get the shape you want.
I've looked at a few apps which will render a 3D image on your computer from G-code, but I don't know off-hand of one which will "decompile" into a STL or mesh file.