I've been wanting to change some textures around for a while, and I have already changed my portal gun texture. However, after looking into customizing the weighted storage cube, I've learned that the file is inside of a GCFScape folder.

I made the custom texture, converted it to a .vtf, and then got stuck. How am I meant to replace the original metal_box.vtf with my custom one? I can't seem to figure out how to put a file into GCFScape, only copy things out.

metal_box.vtf is located in Source Unpack/portal/materials/models/props , you can then just replace it with your custom box.

Is that for the source unpack version or steam? I'm using steam

that is for unpack I believe