I finished my project, how do I license it?

Forgive if this is an obvious question but I'm new to software programming and I'm not familiar with licensing in general. I finished my project using STM32CubeIDE and I want to release my work under GPLv3 license. The issue is that my own code is just what is written inside "USER CODE" areas in the main.c file while all the other code is copyrighted by ST and released under 3-Clause BSD license. What is the proper way to apply GPLv3 to my source? Is it enough to attach a license.txt file to the repository containing something like "every part inside USER CODE is licensed under GPLv3. All the rest is copyright of ST and released under 3-Clause BSD license"?

Thanks for any help, best regards.

Lorenzo