There are lies, damn lies and statistics.
The prognosis is gloomy but ... my 70 year old dad was given six months to live because not only did he have GBM, he also had an anuerysm in his stomach ready to burst and had close to emergency surgery to repair that. And then had complications from that surgery and blood clots and ...
He was given six months to live and although he is now in a hospice, it is 13.5 months later.
At 46 years old, I am not ready to lose my dad so I understand what you feel like. It may sound like a silly clique now but treat each day as a miracle and spend a ton of quality time with your dad. Be realistic that things might not work out but never give up hope. It is all about buying quality time whether that be one more day, week, month, year or decade or more. The one thing that worked for me from day one was to detachment myself - I always was looking at the positive and that this would be the treatment that worked and my dad would be the one who was going to be the 70 year old who lived ten good years.
Another thing that worked for me was that we always had the new normal. No point looking back at what was lost or forward to what might be lost but to live in the present and whatever that present was is the new normal for you. They thought my dad had a stroke at first as his only symptom was the right side of his face was paralyzed. So we worked around the speech - let him finish a sentence that took a little longer than normal to do or turn off the background noise.
And at 70 my dad's bad knees, hips, etc were slowing him down so where once we would play 40 rounds of golf a year, that was down to 4. There were suppers on the special occasions but this disease "forced" us into a quality of time that we would not have otherwise had. One always believes their parents are going to live forever and that hike or whatever will always be there so one puts that off. It is now time to do those sort of things and take the positive out of this.
And do not be afraid or feel it is weak to get professional help. When my dad was first diagnosed, I cried like a baby but I managed. I was focusing on the positive and that new and better drugs were here or just around the corner. But when my dad was admitted to a hospice, despite it being a great thing, I broke down. There is so much support out there because I know I felt as if I was the only child in the world to ever lose their parent. Or to lose them from brain cancer.
Good luck in this journey. And the best explanation I found about what will happen next is you are going to be on a roller coaster ride and the idiot operator will not let you off. Enjoy and embrace the highs and fight through the lows.