[jordigh]

You don't have to distribute your own stuff under the terms of the GPL. Any other GPL-compatible license will work. Most open source licenses are GPL-compatible, so you should have no problem.

Of course, like you say, this only matters if you want to redistribute. For in-house use, everything's allowed.

[ISL]

Furthermore, unless you're distributing binaries with the GSL compiled in, you can use any license you want for your own code. The things you wrote, you wrote. You can distribute those things however you like.

Six months ago, I , like OP, was very confused and concerned about this same issue. One of several discussions that helped me sort it out: https://news.ycombinator.com/item?id=9477840