I patched the gtags-parser/Cpp so that it does not record forward definitions of classes (e.g. "class myClass;"), because in bigger projects forward definitions may be used very extensively to reduce compile times, and gtags would report a lot "false-positive" matches.
I have not come across a disadvantage of this. Perhaps it could be interesting for forward definitions of functions as well.
Anyway, feel free to include the idea in the global sourcetree, if you want.