There were 350,000 US cases reported in 2014 (the most recent data from the CDC), mostly in people under 25. But it's estimated that the actual number of cases each year is around 800,000, since fewer than half of cases get diagnosed.
If left untreated, it can lead to pelvic inflammatory disease, scar tissue, chronic pain, and infertility in women; it can lead to inflammation of the tubes connected to the testicles in men, which could also lead to infertility.
Gonorrhea is transmitted through infected secretions or fluids, so it can be transmitted through vaginal, oral, or anal sex. So if you're having sex of any kind and aren't sure of your STI status or your partner's status, get tested.