A gaseous anomaly is a type of interstellar phenomenon.
In 2290, the USS Excelsior began a three year mission to catalog gaseous anomalies in the Beta Quadrant. The USS Enterprise-A had been equipped with technology to do the same by 2293. (Star Trek VI: The Undiscovered Country)
In 2370, Romulan Commander Sirol claimed the IRW Terix was conducting a survey on gaseous anomalies in the Devolin system, although they were actually searching for the USS Pegasus. (TNG: "The Pegasus")
In an early draft of the script of Star Trek VI, the anomalies were referred as "gaseous atmospheric anomalies", although this was dropped from the final script.