Don't worry about that. The GPL takes effect when you modify or integrate Lilypond in some product you release to the public. A score created by Lilypond is not such a product.
Regarding snippets: While Lilypond is a tool you are explicitely licensed to use personally and professionally, snippets obtained from some random place are usually not licensed specifically. But that would only be an issue if you wanted to distribute such code in some product, as long as you just use it internally for your score no one is going to say something about it. Cheers, Valentin
