Global Navigation Satellite System Reflectometry (GNSS-R) is an active, bistatic remote sensing technique operating at L-band frequencies. GNSS-R signals scattered from a rough ocean surface are known to interact with longer surface waves than traditional scatterometery and altimetry signals. A revised forward model for GNSS-R measurements is presented which assumes an ocean surface wave spectrum that is forced by other sources than just the local near-surface winds. The model is motivated by recent spaceborne GNSS-R observations that indicate a strong scattering dependence on significant wave height, even after controlling for local wind speed. This behavior is not well represented by the most commonly used GNSS-R scattering model, which features a one-to-one relationship between wind speed and the mean-square-slope of the ocean surface. The revised forward model incorporates a third generation wave model that is skillful at representing long waves, an anchored spectral tail model, and a GNSS-R electromagnetic scattering model. In comparisons with the spaceborne measurements, the new model is much better able to reproduce the empirical behavior.