The orbital period ratios of all Kepler planets (blue histogram and blue line) are compared to those of Kepler planets in systems with the gap complexity C  <  0.1 and observed multiplicity m_obs≥4 (black lines). Planets in the low gap-complexity systems are often found near the 3:2 and 2:1 mean-motion resonances (MMRs; vertical dashed lines). These systems contribute to the divergence between the model and observed gap complexities in Figure 4.