I'm not sure how Trimble creates their .ggf files, but on Monday December 9th the NGS is going to announce:
NGS Industry Partners-
I am writing to give you advance notice of a Critical Update to the GEOID18 little endian and ascii grid files. If you or your team downloaded and used either of these grid file types prior to November 26th, 2019 we urge you to replace them with the files currently available on the GEOID18 Downloads page. NGS will issue the public announcement copied below at around 4:00 pm Eastern Time, on Monday, December 9th. Please consider sharing this advance warning with any of your colleagues that may be affected by this update.
I assume these specific files are not the source material for Trimble's .ggf, but I'm hopeful for a little reassurance before I push out some deliverables derived from Trimble's Geoid18 file.