Is it true Gilthunder statement about Meliodas and Ban being the strongest nanatsu no taizai members?
The tricky thing with calling someone the strongest in any show is that it is very quickly opinion based. One could say that Escanor is easily the strongest member of the group because he just obliterates everyone he sees. Meliodas has his full-counter move which is also a feature one could use to call him the strongest. Ban is immortal. He litterally can't die. This feature of Ban could be used to argue that he is the strongest member (he also has the whole muscle buff mode thingie). 

The point I am trying to make here is that (in the way i see it) Gilthunder believes Meliodas and Ban to be the strongest members based on things he knows/has seen. I don't believe that you should take his statement 100% at face value when it comes to actual power level scaling.