Good save—if not caught quickly, diesel can cause extensive damage to an engine designed to take gas.
For a gasoline-based engine, the biggest worry around diesel is its density. As a much thicker substance, diesel can clog your fuel pump system, affecting compression and causing the engine to turn over. Do not start your engine if you’ve put diesel fuel in to avoid this.
Once the vehicle has been towed to a shop, a mechanic can pump diesel fuel out of your tank. While low amounts of diesel (one gallon) likely won’t destroy your engine, it’s best not to drive with any trace of diesel in the tank.