GPL means open source. Means you, I, and Frank down the road can download and read the source. We can compile it ourselves from the source, to make sure we have what we expect. We can inspect the code to discover back doors (hiding a direct back-door in source is REALLY REALLY hard to do) - crypto experts in this or other countries can look through it to ascertain if it is secure or subject to attacks (theses have been based on this, so it is no idle task)
In fact no one with any standing in the cryto community of experts will say any crypto algorithm is anything but *insecure* until the whole world has tried to break it, and all fail.
Good crypto is possible. GPL means open source, so good crypto applications *are* possible. Anything close sourced, i.e. commercial or government furnished, as binary only, cannot be trusted.
That is why GPL is so important. It isn't magical, but no one can muck with it in secret. Just isn't physically possible - you get the source code too.