So you want to make your own props or skins for gmod? Well what about custom textures for source sdk?
I will tell you and IF you still are confused I will also post some urls for youtube videos that help alot of people.
Skins :: So to make a custom skin you must go to…
Now pick the area where the specific model skin is. I will just reskin a ragdoll to start because it is the easiest. Now once you clicked models, you click humans? I dont have my computer with me to see but I remember all of it. MOSTLY. So after clicking humans or people or whatever it is, you then click either group1 or group 2 or male or female. I dont remember what goes first but then click whatever character face you want. I click on lance_headmap and I must open it with photoshop. You should also, now once you open it, it should look like a globe of a face just flattened out. So now just edit the face as you want. Once you are done with that just save it as the same name it started as but save it onto your desktop. Make sure the file type is .vmf and in order to get that you need a plug in. The url is http://nemesis.thewavelength.net/index.php?c=154 and just follow those instructions on how to get the plugin into the photoshop files. Once you save it as you should, just go to the original file located somewhere in the gmod folder and just replace that original with your recreation that is saved on your desktop. MAKE SURE THAT THE FACE IS THE SAME AS THE BODY YOU ARE EDITING OR ELSE IT MIGHT NOT WORK THE WAY YOU INTENDED!
Models :: In order to make models on gmod just follow this tutorial because I am way to lazy to write it all out. PART ONE http://www.youtube.com/watch?v=drMErmG-Xx0
PART TWO http://www.youtube.com/watch?v=JRQ9OyvA_EM&feature=related
Textures :: http://www.youtube.com/watch?v=x3Oy5lxcomA
Hope this helped and I am sorry if I didnt answer your questions. If you want some more help just look for it on youtube or try contacting me at email@example.com or even just send me a message on steam. My steamid is ChewyDuck OR Schwumper