I would say no, because a MATLAB licence is required to run the software, but this point isn't completely clear to me.
EDIT: This was a terminology question, not a request for legal advice.
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The term "free software" (especially when used as a synonym for free-and-open-source software, FOSS) usually refers to the license terms of the software itself, it does not depend on the platform needed to run a program. Otherwise, programs bound to commercial operating system like Windows or iOS could never be called "free software". GPL is widely accepted as a FOSS license, so the answer is clearly yes, a GPL lib is free software, regardless of the platform required for using it.
I would also recommend to have a look into the FSFs definition of free software. They mention four essential freedoms, but none of those points requires the platform for running the program to be FOSS as well. This is obvious for points like reading/modifying the source code, or the right of redistribution, but even the first one - "The freedom to run the program as you wish, for any purpose" does not say "the hardware/software platform required for this needs to be free as well".
Independently from this, note there is a free MATLAB clone called GNU Octave. If someone has the intention to write a free lib which does not require any "unfree" software, they could make it compatible with GNU Octave on Linux. Nevertheless, to my understanding, the freedom of a lib does not depend on the availability of such a free platform.