Is this viable - Gel Batteries ?
No that will not really work as you want it too. You need 4 or 8 of the same.
Although you will initially measure 48V.
Charging and discharging a battery is done by current, and all batteries in the bank must endure the same through current. Or otherwise, you will end up with a combination of some overcharged and some flat batteries within a bank like this.
That picture is very confusing, but I see at least two dead shorts.
The design is still at the conceptual stage methinks.
I have 6 batteries available and a 48V Inverter
besides the dead shorts it should be OK then
Only with 4 batteries
Use 4 batteries and Victron Battery Balancers.