If Punkbuster says this, it means you have pk3 files in your "\sof2gold\base\" folder or in the "\base\mp" folder. Those files can be mods, or weaponskins, or mapskins, or soundpacks, etc. Sof2 GOLD does not allow any kind of skins & soundmods, they are considerd as cheats.
To solve this problem, you have to delete all the invalid pk3 files in you 'base' or 'mp' folder. Start with the 'mp' folder. that's easy, just delete every .PK3 file in there. Every PK3 file in the 'mp' folder has to be deleted. The 'base' folder is harder, as that folder also has to contain valid PK3 files. Below this text you see a list of valid & required PK3 files. You have to delete every PK3 file from you 'base' folder, that is NOT on that list.
Once you deleted those invalid PK3 files, you should no longer have that problem.