IMERG is now available back to June 2000 of the TRMM era. Both the 'early'
and the 'late' products have been retrospectively processed. These products are available in the same location on the jsimpson.pps.eosdis.nasa.gov FTP server as the current GPM IMERG products: /NRTPUB/imerg/early/YYYYMM and /NRTPUB/imerg/late/YYYYMM.
We are in the process of retrospectively processing the GIS-friendly versions and accumulations of these IMERG products. This is likely to be completed by the end of September 2019.