R80.20 is dramatically better than R80.10 on the GW. R80.30 has further improvements over R80.20 but is also newer (like you say). As said above, in some cases like SSL inspection, R80.30 is dramatically better than R80.20. In other cases, you can also choose R80.20.
I do not see a reason to upgrade R77.30 GW to R80.10 by now.
You can stay with R80.10 mgmt and move the GWs to R80.20 or 30 but you can also upgrade the mgmt as you will need it to leverage new functionality later.
If you choose to do mgmt upgrade i would go directly to R80.30 (even though its newer, you want to be able to leverage new functionality).
BTW the audience, as always, is welcome to comment back