Is possible to fix this somehow ?
It could be, but doubt it. You can’t fix it unless Valve looks at it and Garry.
Can’t be fixed, they use gradients in order to save memory. GMod currently has no support for this so you’ll have to wait for Portal 2 (hopefully with gradients and the Infected shader). For more information please refer to this file http://www.valvesoftware.com/publications/2010/GDC10_ShaderTechniquesL4D2.pdf, however you have to save the link as a PDF file before you can read it. Use Adobe Reader