Hello Old Sailor,
The significance of the Gleason score is that it is a measure of the aggressiveness of the cancer. Higher score equals more aggressive. The more aggressive the cancer the more likely that there is or may be metastasis. Metastasis may be too small to pick up by CT or bone scans at an early stage. I had at least one doctor tell me that in my case it was highly likely that there was micro metastasis. This was due to my high PSA value coupled to the high Gleason. If you are familiar with the Parton tables and you have your numbers you can go the Sloan Kettering site and plug in the data and see what the difference is for your case versus a hypothetical case. Case 1. CS 4+4, PSA 5.0 Case 2. Gleason Score 3+3, PSA 5.0.
Do a google search on Gleason Score and Prostate cancer and you will get far better and more detailed information than I have given here.
Best wishes for a rising tide and a fair wind.