Being alive means certain death. Life always ends with death.
Glioblastoma appears to be an aggressive cancer according to this web site http://www.abta.org/understanding-brain-tumors/types-of-tumors/glioblastoma.html?gclid=CJ3d56eIxbQCFYVFMgodGiQAXw but that's not the same thing as "certain death." I note that they cite a 2009 study showing a 5 year survival rate of 10%. That means that the data are based on treatments 8 or 9 years ago. This is a problem with published survival data. And, there's nothing that tells you how long you have. Your oncologist may have a better estimate--but I'd want to see what progress treatment makes before taking that as more than an educated guess.
Several people here have beaten 5% survival rates. Everyone is an individual and everyone responds differently to treatment. Fight this thing and see how it goes.
Also, a second opinion from a major cancer research hospital might be in order.