Nucleotidyltransferase that catalyzes the formation of cyclic GMP-AMP from ATP and GTP and plays a key role in antiviral innate immunity (PubMed:34261128). Directly binds some unknown nucleic acid, activating the nucleotidyltransferase activity, leading to synthesis of both 3',2'-cGAMP and 2',3'-cGAMP second messengers (PubMed:34261128). 3',2'-cGAMP and 2',3'-cGAMP bind to and activate Sting, thereby triggering the antiviral immune response via activation of the NF-kappa-B transcription factor Rel (Relish) (PubMed:34261128). ATP + GTP = 3',2'-cGAMP + 2 diphosphate ATP + GTP = 2',3'-cGAMP + 2 diphosphate The enzyme activity is specifically activated by some nucleic acid. Belongs to the mab-21 family.