Mounting and Content question

If I get a custom skin for my mounted counter strike source, will I also see the same skin on gmod if I use something like CSS realistic weapons?

If you put the skin into the GMod folders you will.

so getting a skin for my counter strike and gmod is completely different?

No. IT is just that any custom stuff for CSS is not automatically put into GMod, because GMod only uses the files in the .gcfs, which have all the files that Valve made in them.

Yes great.