Sorry to hear about your dad. This is still being researched. In fact, there's an ongoing study listed on
clinicaltrials.gov which is basically a survey of relatives of two or more GBM patients. For many years, the received wisdom was that GBM was not hereditary-- so much so that an 80-year-old doctor assured me the other day that it couldn't possibly be. However, there have been several case studies published recently... familial glioblastoma seems to be rare but not unheard of. It may be that it is most commonly a genetic condition (such as NF1) that is inherited, which in turn increases the likelihood of GBM. There are several such conditions listed in the wikipedia article on GBM. --("Brain trauma" sounds like a head injury, though, like from an accident.)