There is a family of viruses called the herpes viruses, and they include the chickenpox virus (also causes shingles), herpes type 1 (causes most fever blisters), herpes type 2 (causes most genital ulcers), and cytomegalovirus. The herpes types 1 & 2 are only in the affected skin and the nerves from the skin to the spinal cord, the herpes viruses are not in the brain. Folks with herpes type 2 and the genital ulcers do not have cytomegalovirus as the cause of the genital ulcers.
With 80% of the population having cytomegalovirus in the brain and only 10,000 patients with GBM out of the 300 million US citizens, seems like the link is doubtful.
The situation with cervical cancer and human papilloma virus is very different. Human papilloma virus produces precancerous changes in the cervical skin that are easily seen on pap smears years before these skin changes develop into cervical cancer. I haven't seen anything suggesting that there is a precancerous phase to GBM.