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They are the successors to GCFs, and when passed to the addoninstaller tool are also used to distribute add-ons like custom campaigns.VPKs can be created with the command line tool vpk, found at bin. These are files that are uncompressed archives used to package content (never code). How to create a custom portal gun skin for Portal 2 (since the other guide got taken down I had to re-figure it.
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