There is a plugin somewhere from NIVDIA for making normal maps (the actual term, but can also be called “bumpmaps” which is how they are reffered to in the VMT, i’ll come to taht in a min) in Photoshop, it is a filter, which I think takes the lighting in your texture and converts upward looking parts to an upward “emboss” (for want of a better term) and downward looking parts to a downward “indent”.
This is then read by the program you use it in (in this case it will be GMod, but you can use them for renders aswell and in other games like UT3) and it makes the parts that look embossed stick out and the indented parts look indented. This is a technique used to allow you to have the illusion of a high detailed model on a low detailed model (which is another way you can make normal maps but its much more complex).
I’ve said about the Photoshop filter plugin, but personally I use the GIMP one (mainly because I don’t have Photoshop, but I believe also, from what i’ve seen, the quality in the bumpmaps produced in the GIMP one is better than the photoshop one, but I could have just seen crap Photoshops ones :P).
Ok, to implement a normal map (after you’ve made it), you must open the VMT for your texture (you do know what a VMT is right?) and it will likely look like this:
(for the most basic texture with no extra parameters, also note I have not put “quotation marks” around the parameter name as it is not actually needed but some people do still put them there)
Ok now to add your normal map you must convert it to a VTF and name it how you want (but i’d advise something like “textureitsfor_normal” so youknow what it is.
Now you must get it so GMod knows to use your normal map to do this you add the line $bumpmap to your VMT like so:
This should now correctly (if you put the path right and spelt the name right :P) reference your normal map for the texture.
Hope that makes some sort of sense to you and you now understand what normal maps are and how to implement them in GMod (or other Source games)