I heard that you must document all your changes and modifications to a software under the GPLv3. Is this true? If I'm not distributing this software, do I have to document my changes/modifications. How are you supposed to do this? Thanks in advance!
You may copy, distribute and modify the software as long as you track changes/dates in source files. Any modifications to or software including (via compiler) GPL-licensed code must also be made available under the GPL along with build & install instructions.
From tl;dr legal