what is the license for the Wireshark GUI?

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And this is the license for almost all of Wireshark (with a few exceptions, such as the Internet checksum code, which is BSD-licensed), so there's very little of the Wireshark code you can use in a program or library without making your program or library GPL, so that the source code must be made available.

Here's the GPL v2, and here's the FAQ for the GPL v2, which says a lot about what you can, and can't, do with GPLed code.