All GHA are regularly audited by the airlines they are contracted to handle. With some airlines for a busy route it can be annually, others might make it bi-annual or share an audit. One aspect that the auditors will target are the training records of the ramp staff and especially those operating the push back tugs, Hi-lo's etc.
A push back tractor driver will also have a basic training in RT procedures as they will be inter acting with ATC.
Many GHA have printed licences that can be ramp checked and these allow auditors to confirm that the operator has the right training and is up to date with their re-currency training.
Even the humble baggage tractor drivers have to be trained and their licence stamped before being let loose on the ramp.