All the playermodels in Garry's Mod and other Source games have a specific skeleton organization.
The Valve standard skeleton is named ValveBiped and all the bones have specific names and properties to be recognized as a Valve Biped model.
Everything starts with a ValveBiped.Bip01_Pelvis, it will be the root bone of all the model's skeleton. Every other bone in the hierarchy should be parented directly or indirectly to this bone.
Upper body bones structure