It uses CRISPR technology, which is not what is usually meant by GMO. Usually GMO inserts a gene from another organism. CRISPR allows you to insert DNA sequences precisely. Combined with another technology called gene drive (which I don’t really understand) what it does is cause the new gene to spread through the entire population (of mosquitoes) since individuals with only one copy are sterile.
What I read about all this is that they have bred strains of mosquitoes that cannot carry malaria and are awaiting permission to release them. If they have made strains that cannot carry zika, I haven’t read about them, but perhaps they have.
I believe they were working on strains that couldn’t carry dengue fever, a close relative of zika. But there are four strains of dengue and you can get all four, so they want to make the mosquitoes immune to all of them.
As mentioned above, this technology does not reduce the incidence of mosquitoes, only renders them incapable of carrying that particular disease.