Gustaf is described as 'The digital alternative to sheet music. Manage all your music with the Gustaf App. Buy interactive and 100% legal scores in the Gustaf Store. Supports PDF and interactive digital music scores' and is an website in the Audio & Music category. There are four alternatives to Gustaf, not only websites but also apps for a variety of platforms, including iPad, Mac, Windows and iPhone. The best alternative is MuseScore, which is both free and Open Source. Other great sites and apps similar to Gustaf are Noteflight (Freemium), Instrument Bible (Paid) and Chromatik (Free).
Noteflight is an amazing online notation editor. On the free version you get 10+ instruments to choose from, lots of musical symbols and more. On the payed version (Cresendo) you get lots more instruments, guitar tab and sharing options. Best way to wite music on the web.