I'm not doctor, but having done much research, here is what I have understood. Gleason Scores get worse over time. PCa is generally, but not always, a slow growing cancer, so the docs don't seem to get too emergent about doing treatment immediately after diagnosis, but recommend not waiting more than 2-3 months.
Gleason Scores identify the progression of the cancer cells from being very differentiated which is typical of normal cells (low Gleason Scores) to being undifferentiated which is typical of advanced cancer cells (high Gleason Scores). As the cancer progresses, the cells become increasingly undifferentiated. If you look up Gleason Score on the Internet you will find pictures that show how the cancer cells appear to be broken apart instead of circular as you would expect of normal cells.
Also, the more time the cancer has to progress, the more the risk that the cancer will move beyond the prostate into surrounding tissues and other areas of the body (metastasis). Once this occurs, methods of treating prostate cancer locally (i.e., surgery and radiation) are no longer effective.
So doing something sooner is suggested to be better by many professionals. To provide a complete picture, however, some advocates recommend watchful waiting, because prostate cancer is so slow growing that it may ultimately not become the cause of death in an individual when other health issues may become more serious and fatal themselves. This argument becomes increasingly stronger with more advanced age. However, regardless of age, the risks of watchful waiting are highly debated by advocates of early detection and treatment.
Most folks here that I have observed have opted for treatment soon after diagnosis as the progression of cancer is scary, and if it does spread is more difficult to treat.
Hope this information is helpful.