He worked to develop gamma-ray astrophysics from a field of experiments detecting a few objects to a full astronomical discipline with thousands of sources in many classes.

He worked predominantly in the area of high energy astrophysics, studying explosive objects in the universe such as supernovae, gamma-ray bursts, gravitational wave transients and tidal disruption events (stars torn apart when they approach too close to a massive black hole).

The Gehrels and Stone paper describing the measurement made a prediction, later confirmed, that the auroral emissions on Jupiter are cause by precipitating oxygen and sulfur ions.

As a postdoc and then permanent scientist at Goddard, Gehrels worked on the GRIS balloon payload for high resolution (germanium detector) spectroscopy of gamma-ray sources.

The GRIS detection of a broadened and redshift 847 keV line was some of the first strong evidence for mixing and asymmetries in the ejecta, now recognized as signature characteristics of supernova explosions.

Discoveries include an isotropic distribution of gamma-ray bursts (GRBs) on the sky, supporting an extragalactic origin, two classes of GRBs with short and long durations, blazars with bright gamma-ray emission and harder spectra than Seyfert AGN (e.g. Dermer & Gehrels 1995), detailed mapping of gamma-rays from Al-26 decay in the galactic plane mapping regions of nucleosynthesis over the past million years, and detailed mapping of the 511 keV line from positron annihilation in the galaxy with concentration at the galactic center.

Since approximately 2009, it has become a community tool for observing transient and variable sources of all types including novae, supernova, AGN, magnetars, galactic black hole and neutron stars, tidal disruption events, and comets.

The mission has characterized the afterglow and origin of short GRB for the first time, determine the shape of the X-ray and optical afterglow lightcurves to great precision and with large statistics (>1000 GRBs), provided a comprehensive data set of UV observation of supernova, and discovered X-ray outbursts from supernova shock breakouts and relativistically beamed tidal disruption events.

For Fermi, Gehrels worked with lead scientists at Goddard, Stanford, the United States Naval Research Laboratory, and several other institutions to propose the mission and bring it to fruition.

It has revolutionized our understanding of the high-energy gamma ray sky with observations of pulsars, AGN, GRBs, novae, and diffuse emissions.

Gehrels joined the SNAP dark energy mission proposal led by Saul Perlmutter and Michael Levi in 2008.

This evolved in the DOE-NASA Joint Dark Energy Mission (JDEM) program, for which Gehrels became the project scientist and chaired the Science Coordination Group in 2009.

Gehrels was the project scientist and chaired the Formulation Science Working Group with co-chairs Jeremy Kasdin and David Spergel.