I was wondering if anybody had heard of a library, preferably a .NET assembly, but Java will do as wel, that allows you to read the data in a Guitar Pro file (.gp3-gp4-gp5)
TuxGuitar is an open source Java application that includes classes that read Guitar Pro files. If should be probably more that sufficient for reading basic metadata.
Your other option is using reverse-engineered file format documentation - there's one from DGuitar project.
Your third option is trying to re-use some code from KGuitar project, that also include Guitar Pro 3/4/5 files importing clasess, but it's in C++ and Qt.
Please look through my open source project that is written in C# for Windows Phone7:
http://phoneguitartab.codeplex.com/
This is tablature viewer for Windows Phone 7. It can open guitar pro (.gp3, .gp4, .gp5) and text based files (I've used open source projects in Java to create guitar pro parser in .NET). Unfortunately it isn't finish yet but I hope you find something useful in this project.
Update: GuitarPro functionality was moved into branch