You can assign whichever license you wish to your code, regardless of where you put it. Lots of software use the BSD license, it's not the property of Mathworks.
To assign the license to your code, the only thing you'd have to do is just to state it. You can do that any way you want, via a license.txt file, a header in each source code file.
Note that the BSD license and the GNU GPL license are fundamentally different. With BSD anyway distributing your code with theirs only has to include your copyright notice, with GPL they also have to include your and their source code. There are plenty of other open source licenses to choose from as well. If you do really care about the license, you need to seek the advice of a lawyer.
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