Killing Floor 2 is done with Unreal Engine 3, so you don't need to do anything more, then doing a linear interpolation between your glow color and black, based on a mask texture, and to plug that into the emissive channel.
K so I took a look at the video that you linked, and what he does, is to put the mask texture into the alpha channel of the reflectivity map of the weapon. So probably you get the entire gun glowing, because your alpha didn't got saved. Open up your texture in your image editor, and check if the alpha channel is still…