We show that a model based on anticrossing between highly group velocity-mismatched gap-guided and index-guided modes describes gap soliton propagation in photonic crystal waveguides. Such nonlinear solutions can be exploited for exploring new regimes such as all-optical control of group velocity (dispersionless slow light) over a submillimeter length scale, and propagation beyond the linear modal cutoff. The results are validated by means of finite-difference time domain simulations.