Giving you a complete answer is difficult as you didn't provide any additional info as to how you plan on charging, maintaining and monitoring your new system.
I'm not sure and don't know if the L16's you are planning on are wet cell or AGM. GC batteries are usually wet cell.
You also don't provide any info on your boat size.
And you don't mention your charging and inverter setup.
I had a house bank put in our boat in '16. Previous to that we had 2 4D wet cell batteries which we used, not very well, for house & engine with a 3 way switch to utilize the batteries.
The new house bank is 6 L16 6V AGM batteries. These batteries are very heavy and I was able to remove a set of 'day tanks' that were in the bottom of the bilge and put the batteries there, low center of gravity. After these were put in I also needed to upgrade the charging system. The OEM alternators (alts) did not have the output to handle this new load. New Balmar alts 120A were put in. This also required 2 (twin engines) new regulators, duo-chargers and a centerfielder (all Balmar). The centerfielder controls where the power goes. Whether to the house or engine bank.
As I said I also kept the 2 4D wet cell batteries, which were utilized as an engine bank.
Our Xantrex, SW3012 charger/inverter has a 3 way charging system which is very important in properly maintaining your new intended battery banks. I set the charging profile to AGM as they were the most valuable part of the system of batteries. This meant that the wet cells were not being charged properly. In this instance I knew that and knew I would be swapping out the wet cell shortly, which I did at the beginning of this year.
Any charger you have should be able to Bulk, Absorb & Float charge your batteries.
A good inverter should be part of your setup. An expenditure on a good battery bank can only be utilized with a good inverter. Our Xantrex also has a control panel which enables us to 'see' exactly what's going on with the charging and discharging of our house (L16) battery bank. I am adding a separate monitoring system for the engine bank.
I would agree, with some of the posters, that combining, in a single bank, is NOT a good idea.
So, if you'd like to contact me off-forum I'd be glad to further discuss what I've done and how it could be applicable to you.