Experiments were performed on the 100-J class Optical Sciences Laser at LLNL to characterize the saturation fluence and small-signal gain of a solid-state Nd:glass amplifier utilizing LG-750 and LG-770, an amplifier glass developed for the National Ignition Facility (NIF). These high quality measurements of gain saturation at NIF level fluences, i.e., 10 - 15 J/cm2, provide essential parameters for the amplifier performance codes used to design NIF and future high power laser systems. The small- signal gain, saturation fluence and square-pulse distortion were measured as a function of input fluence and pulse length in platinum-free LG-750 and LG-770. The input fluence, output fluence, small-signal gain and passive losses were measured to allow calculation of the saturation fluence. Least squares fits of the output vs. input fluence data using a Frantz-Nodvik model was used to obtain an average saturation fluence for each data set. Overall, gain saturation in LG-750 and LG-770 is comparable to long pulse lengths. For shorter pulse lengths, < 5 ns, LG-770 exhibits a stronger pulse length dependence than LG-750, possibly de to a longer terminal level lifetime. LG-770 also has a higher cross-section, which is reflected by its slightly higher extraction efficiency.