I just looked up how the gel works. It's a progestin (synthetic progesterone) plus testosterone.
Progestin does the same as the GnRH hormone therapy drugs (Zoladex, Prostap, Decapeptyl, Degarelix), but not as well and it's cheaper. This will shut down the two hormones produced by the pituitary gland which are used to signal the testicles:
Luteinising Hormone (LH) is used to tell the testicles to produce testosterone.
Follicle Stimulating Hormone (FSH) is used to tell the testicles to produce sperm.
(I cover this in detail in my How Hormone Therapy Drugs Work lectures.)
This will stop sperm production, but will also stop testosterone with all the associated symptoms we know only too well here.
So adding Testosterone too will will replace that which isn't being produced by the testicles anymore.
To answer your question, no this wouldn't be any use for prostate cancer treatment, as the added testosterone will mean the prostate cells are activated as normal. I also wonder how well controlled the resulting testosterone levels are. I could imagine some men using it to excess as a form of testosterone replacement therapy to boost their testosterone.