I had this problem too,l what happened is that the axis of the bones are set to z+ (z axis up, blender default). select the armature in edit mode hit CTRL+N and select Y+ (also hit a to select every thing). it looks fine in hammer because it probably has no animations, once in Garry's Mod it adds animation with the Y+ axis up and the model and source have no clue how to respond, causing that mess of your model. I hope this helped, I been using blender for about a year now and i found this out yesterday.

i fixed it the bones need to be set with +Y as the axis in blender by pressing CTRL+N and selecting global +Y axis

I have used these tutorials before, in fact they are what got me using blender and modding HL2

Thanks

Whenever I compile a v_model in hammer that I made in Blender with Blender Source Tools, it comes out messed up. Distorted mesh incorrect movements (moving up instead of rotating) and incorrect placement are most of the major problems.

I have been able to reskin models using the default animations, but whenever I make custom animations the don’t work as mentioned above. I have messed with all the setting and it makes no difference (I have tried .smd instead of .dmx). Currently I am using the Gmod rig, and a custom USP 45 Match model. This problem occurs when I am using different rigs.

I’ve been at this for about a month and have made zero progress, I am super frustrated, pleas any suggestions would help.

Thanks