FWIW all the GHA I have had dealings with have had recorded scheduled maintenance for their ground equipment, and the service records are available for the audits by all the airlines they handle and the Airport authorities. The airlines might not have their own ground equipment qualified personnel, although some do, but their audit departments do visit the GHA's and they do check the records and also look at equipment working on their aeroplanes.
The airports will have their own vehicle maintenance departments looking after their airside ground equipment, and yes they have to audit GHA operating on their airport.
It isn't as mickey mouse as some on here seem to think.