Google "are ideas copyrightable". Copyright protects the specific, tangible expression, not the idea or concept itself. So in your case, having "got ... ideas

Google "are ideas copyrightable". Copyright protects the specific, tangible expression, not the idea or concept itself. So in your case, having "got ... ideas by looking at the source code of a GPL project" doesn't necessarily mean there is any violation of copyright. Employers will often require that employees haven't seen sensitive code at all (clean room) as it avoids any possibility of copying the code itself and provides powerful evidence if they're ever sued, but that's not a legal requirement, just a cautious practice. To read code, learn a concept from it and then apply that concept in your own code is fine (as far as copyright is concerned) as long as you're not copying the code itself. I am not a lawyer and the above is not legal advice, ofc :-)