We developed a Satpy reader for the GERB HR data product. Please find it at the following URL: https://github.com/pdebuyl/satpy/tree/add_gerb_l2_hr_h5 (feel free to ask Pierre de Buyl - email pierre.debuyl AT-sign meteo.be) if you need more information for configuration).
An example notebook for reading and resampling the data is available: Jupyter file and PDF preview