Gecko is a
layout engine:
http://en.wikipedia.org/wiki/Gecko_%28layout_engine%29[
^].
It means, the functionality related to the browser windows, their activation and layout is implemented
outside of Gecko, in a browser using Gecko or other engine.
Your question does not really make sense, in the following aspect: Firefox is already created. Internally, it already has all the required mechanisms to deal with pages, and so on. You really need to describe the type of application which could possibly collaborate with existing Firefox, but I doubt it may make sense. Anyway, if you wanted to use Gecko, you would implement your own browser with features you need.
And the second part of the question, about a wrapper, makes even less sense:
Gecko is already written in C++, so no wrapper is needed. Please see the article referenced above.
—SA