One interesting Gir crossover Invader Zim fanfiction could be one where Gir teams up with Dib to stop some new alien threat that neither Zim nor the other regular villains can handle. In this story, Gir's unpredictable and silly nature might add a lot of comical elements to the usually tense relationship between Dib and Zim.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for Gir crossover Invader Zim stories. Just use the search bar and type in the relevant keywords.
Often, there's a theme of self - discovery. As Female Zim is a different take on the character, she might be on a journey to figure out who she is in this new form. Also, the theme of adaptation is common. Just like in the original where Zim has to adapt to Earth, Female Zim may have her own unique ways of adapting, whether it's to Earth's environment or to the reactions of the other characters around her. And there's also the theme of power. How does being female change Zim's perception and use of power? This is something that many fanfictions like to play with.
It depends on how you look at it. Some fans consider the comics canon because they continue and enrich the storylines from the show. However, others might have reservations based on personal interpretations.
Well, it's a bit hard to give an exact number. Different publishers and creators have come up with various Invader Zim comic offerings over the years. You might have to do some research to get a precise count.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created works, including many gir x zim fanfics. Just use the search function on the site and type in 'gir x zim' to find relevant stories.