I want to incorporate an open source library that is GPL-3.0 licensed into my program. The library is quite big in size and I would be just fine with just a small part of it. Will removing most of its code and then using that version with my software count as modifying the original library's source code? (e.g. Will I have to distribute the source code of the library I "modified", which is basically the same with some classes removed).
Per the GPL-3.0 license,
For both users' and authors' sake, the GPL requires that modified versions be marked as changed, so that their problems will not be attributed erroneously to authors of previous versions.
To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.
In summary, modify is defined as anything other copying as-is; truncating a library would be considered modifying it, and therefore, requires that the adapted version be marked as such.