Except in the case of the GPL license, your rights to reuse and redistribute the source code are contingent upon your derived work also being licensed under the GPL. This is not a hypothetical untested legalese requirement but a real legal requirement that's held up in the courts of several countries.

So yes, using a substantial portion of GPL code in your proprietary software product is copyright infringement. Or even using a substantial portion of GPL code and not licensing the code under an appropriate license - https://www.gnu.org/licenses/gpl-faq.html#WhatDoesCompatMean