This might be an odd question to ask, but I want to be sure to understand it correctly. When I clone a project in Github that is licensed under GPL-3.0 license, am I allowed to delete bunch of files that I no longer need?
Someone took an old BCX-BASIC program and converted it into C with some additional work to make it compile in Linux, and a Python GUI. The original Basic program is still included in the project. I want to delete everything besides the .c program, license and an include file. I want to add my own Makefile and just create the commandline executable with gcc. And I might change the name of the application too and either create my own GUI or maybe (if its possible) to integrate the C program in Rust or Zig in example, but have to explore this more.
But the files that were included there, the legacy BASIC program and some other files, are licensed under GPL-3 too. Am I allowed to just delete those files from the project?
Yes, the GPL allows you to make modifications. The GPL still applies.
The cloned repository still holds the history and deleted files. The files that are still there retain the GPL. If you make additions to the GPL sources, the GPL applies to those too. (Copyleft license.)
You can check the summary of GPL on ChooseALicense to understand your rights and conditions a bit better.