This can be caused by coming from a server with custom models. Make sure you are joining right after starting up GMod. If the problem continues, please go into your console and post any errors you see.

I did as you said and here are the errors that were in there "$halflambert" is multiply defined in material "models/sentry/uh1d/uh-1d_d"!
Error! Flag "$halflambert" is multiply defined in material "models/sentry/uh1d/uh-1d_e"!
C

Error Vertex File for 'Humans\Group01\Female_02.mdl' checksum -1869278011 should be 1622706653
E

600426019 should be 1349223412
Error Vertex File for 'Humans\Group01\male_06.mdl' checksum 1600426019 should be 1349223412
(

both the female a male errors would be spammed into the console when ever i looked at a civilian