In various vehicles and RVs I use flooded, gel, and AGM batteries. None is the correct choice in all applications.
You wouldn't be able to lift a single 12v AGM that has the ah of two 6v GC2.
I have an AGM in my motorcycle because the battery is under the fuel tank and very difficult to access.
I have gel in the Aliner because they are under the dinette seat. Either gel or AGM would work in this case.
I use flooded 6v GC2 in the Weekend Warrior because they are outside on the tongue and give maximum ah per $. Off-gassing is not an issue with outside batteries.
OP, your particular application and needs determine what is best for you. What other people use may not be relevant.