Consider wrapping library's code in a namespace

[zenden2k]

It is a common practice for c++ libraries to avoid naming collisions.
For example, CDocument is a very common class name in C++ projects.
It could be namespace GumboQuery { or something like this.

[micfan]

Right, but that name is too long...

And, I prefer a Boost style namespace use lower case letters, such as gQeury::doc

[TechnikEmpire]

There's no need to shorten the namespace for user preference. That's why we have features like namespace short = some::long::possibly::nested::namespace; or namespace mypref = GumboQuery. :)

Namepace ought to be unique and spell out explicitly at full length what the unique namespace is.

[TechnikEmpire]

Also imo this is something so simple, somebody could do this in 2 minutes and create a pull request instead of throwing the burden on the repo maintainer.