I'm 32 weeks pregnant and have recently developed gingivitis only around one tooth. Anything to relieve swelling or be concerned about?

Salt water rinse. You might want to try some salt-water rinses to calm down the swelling and help with the soreness. Warm tap-water with as much salt as will dissolve in it. Rinse and spit, do not swallow or you will vomit! And see a dentist asap.
Could be abscess. It is unusual to have "gingivitis" occuring around just one tooth....Your symptom suggests a possible dental abscess and should be evaluated, more so because you are pregnant. So, see your doc or a dentist....As any infection may complicate your pregnancy. Congrats on the pregnancy and good luck.
See your dentist. During pregnancy, hormone levels in the gums are elevated and it is really important to have your teeth professionally cleaned at regular intervals and often even more frequently than when you are not pregnant. The need varies from woman to woman.