The Gemini Multi-Conjugate Adaptive Optics System is the first sodium-based Multi-Laser Guide Star (LGS) Adaptive Optics system used for astronomy. It uses five LGSs distributed on a 1 arcmin2 constellation to measure and compensate for atmospheric distortions. In this Letter, we use the LGS wavefront sensor data to derive the amount of differentialfocus caused by horizontal sodium layer structures, which we call the focus excess (because it comes in addition to the turbulent part). Based on data accumulated at a rate of around 1 week per month during a period of 1 year, we derive an upper limit of the focus excess of 15 ± 60 nm rms for an 8-m telescope over an 85 arcsec baseline. No anisotropy in the spatial distribution of this error is detected. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.