Ok, before I start, I'm gonna say that I DID NOT forget to back up the regular skin when changing the portal gun, in fact, I haven't even changed it yet.

In my Steamapps/common/portal2/portal2/materials/models folder, there is no folder for 'weapon' so I can't save the original copy, because I can't find it. I'm gonna try and get the new skin on now, but I'd appreciate it if anyone could get me a download for the normal skin just in case. Thanks!

The skin is in your pak_01.dir;
It's packed in so you won't overwrite it if you place a new portal gun skin in materials/models/weapons

Wouldn't it be a good idea to simply verify the integrity of your game cache files? I guess this way, the regular skin will be placed again in its right place...

Ok. So I put the skin folders in and merged it. Where will pak_01 be? Did I overwrite it?

If you are doing this manually (not so recommended though), the pak01_dir is located in your "steamapps/common/portal 2/portal 2" folder. This is a .vpk file (pak01_dir.vpk) so it must be compressed in .vpk... After being sure yourself that ALL in a folder called pak01_dir, this time WITH your regular skin folders in it, find a program calles vpk.exe located into common/portal2/bin folder. Then simply drag the folder onto the vpk.exe program and it'll automatically make the pak01_dir.vpk file for you...

Now, cross fingers, and copy/paste (replace) the pak01_dir.vpk file into common/portal2/portal2 folder.

facepalm I already did it earlier, and I think I may have replaced it then.