If you build Julia with the USE_GPL_LIBS=0 flag … then there are no GPL-only libraries used (some are dual-license, as we discussed in the other thread).
The Julia bundles that you can download right now, provided by the community on julialang.org, include GPL libraries, so they are provided under the terms of the GPL – you have the right to source code (obviously we give that to you); you have the right to redistribute the source code; you can’t take those rights away from the next person to whom you give either the code or derived binaries.
The GPL libraries are included because for now it is more important to have those functions available immediately to the end-user in a single download. In the future we might not (there has been work in this direction). But the Julia core code is always MIT licensed if you are using it alone (just because we happen to distribute a GPL bundle now, the license of the bundle doesn’t “flow backward” into the code in the GitHub - JuliaLang/julia: The Julia Programming Language itself).
See this thread, especially the top post and this one.