We demonstrate efficient diffraction of intense ultrashort laser pulses using optical-field-ionization induced plasma-neutral gratings formed by spatially structured ionization of a neutral molecular gas in the interference field of two femtosecond pump pulses. The transient refractive index modulation of the plasma structure persists for at least 10 picoseconds and is used to diffract intense femtosecond signal pulses into the 1st order of diffraction with an average efficiency of up to 35%. Plasma gratings are shown to provide stable diffraction at signal laser intensities greater than 10^14 W/cm^2, exceeding the damage thresholds of conventional solid-state optics by more than two orders of magnitude, continuously for hours at a 10-Hz repetition rate. The experimental diffraction efficiency scales with the grating aperture allowing for a larger millimeter-scale plasma optic, increases with the pump energy and electron density, and reaches a maximum at a specific grating length in agreement with the coupled-mode theory for periodic media. These results demonstrate the scalability, tunability, and high damage threshold of transmissive plasma-based photonic structures, opening new prospects for controlling multi-petawatt laser beams.