The gyrokinetic toroidal code (GTC) is a massively parallel, particle-in-cell code for turbulence simulation in support of the burning plasma experiment ITER, the crucial next step in the quest for fusion energy. GTC is the production code for the multi-institutional US Department Of Energy (DOE) Scientific Discovery through Advanced Computing (SciDAC) project, GSEP Center (Gyrokinetic Simulation of Energetic Particle Turbulence and Transport), and DOE INCITE project that was awarded 35M hours of CPU time for 2011. Currently maintained at UC Irvine, GTC was the first fusion code to reach in production simulations the teraflop in 2001 on the seaborg computer at NERSC and the petaflop in 2008 on the jaguar computer at ORNL. GTC simulation of the turbulence self-regulation by zonal flows was published in a 1998 Science paper, which has received the most citations for any magnetic fusion research paper published since 1996.