Headcanon that Gruber became Herbert's adoptive father after the chemical fire killed his parents. HOWEVER, unable to swallow his pride, Herbert constantly referred to him by his full name or his title, never 'dad', until Gruber's untimely death.
That is until during a heated debate in front of the class about Hill’s plagiarism of Gruber's work, Herbert gets too emotional and, in a moment of vulnerability, slips up by referring to Gruber as "his father," for everyone to hear.