IANAL
If you release your application under GPL, that doesn't mean you can't also license it in other ways, too.
If you're concerned about other code you've incorporated into your application, though, if it's licensed under the GPL, it basically prevents you from licensing it under anything but the GPL. But if the code you've incorporated is just BSD licensed, that doesn't put any real restrictions on you.
Anyway, it all depends on the licensing of the code you've incorporated from elsewhere.
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Bitt Faulk