The source of whatever falls under the GPL must be made available (included with the product, download link, "write us a mail and we send you a CD", doesn't matter). Which parts of a product fall under the GPL depends on how exactly it "contains a GPL part".

E.g. When you use a patched version of Linux in your router, then you'd need to make those patches / the patched code available, but you would not need to make the code of your web GUI available.

If you are using libviralgpl.dll in your ProprietarySecrets.dll though, your entire application becomes a subject of the GPL.