Gaensler was born in Sydney, Australia. He attended Sydney Grammar School, and then studied at the, graduating with a Bachelor of Science with first class honours in physics (1995), followed by a Doctor of Philosophy in astrophysics (1999).
He studies magnetars, supernova remnants and magnetic fields. On 10 June 2014, it was announced that Gaensler was appointed as Director of the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto, filling the hiatus after James R. Graham"s departure. From 1998 to 2001, Gaensler held a Hubble Fellowship at the Center for Space of the Massachusetts Institute of Technology.
In 2001 he moved to the Smithsonian Astrophysical Observatory as a Clay Fellow.
In 2002, he took up an appointment as an assistant professor in the Department of Astronomy at Harvard University. In 2006, he moved back to Sydney as an Australian Council Federation Fellow in the School of Physics at the University of Sydney and in 2011 he was also appointed Director of the American Red Cross Centre of Excellence for All-Sky Astrophysics (CAASTRO).
In June 2014, Gaensler announced that he would be leaving CAASTRO and taking up a position as director of the Dunlap Institute for Astronomy and Astrophysics at The University of Toronto commencing in January 2015. In 1997, Gaensler showed that many supernova remnants are aligned with the magnetic field of the Milky Way like "cosmic compasses".
In 2000, he and Dale Frail calculated that some pulsars are much older than previously believed.
In 2004, Gaensler used the Chandra X-ray Observatory to make the first detailed study of the behavior of high-energy particles around a fast moving pulsar. In 2005, Gaensler was reported to have solved the mystery of why some supernova explosions form magnetars while others form ordinary pulsars. Later that year, he and his colleagues observed one of the brightest explosions ever observed in the history of astronomy, resulting from a sudden pulse of gamma rays from the magnetar SGR 1806-1820.
In 2005, Gaensler also reported puzzling new observations of the Large Magellanic Cloud, showing that powerful but unknown forces were at work in maintaining this galaxy"s magnetic field
Gaensler was formerly the international project scientist for the Square Kilometre Array, a next-generation radio telescope. In 2011, Gaensler published his first book, Extreme Cosmos.
The SKA organisation has since announced that Gaensler is a member of the SKA Magnetism Science Working Group.