I don't think anyone is saying dosing manganese =Goni success
Of course there's lots of other factors. What we know is that Gonis in general are just really finicky and a hard coral to raise in our tank environment. I think having close to ideal water params will go a long way to giving gonis a fighting chance at survival in our tank (this applies to all corals in general). Ideal water params means having some level of manganese. Since manganese depletes very quickly, there's a very good chance most people's manganese level is too low unless your dosing it in some form, hence the general dose manganese advice.
Granted some gonis like the red varieties are just about bullet proof and can survive in just about any condition. Without knowing the particular demands of the OP goni variety telling him to get an ICP test to get his water params close to ideal and pay close attention to manganese level is good advice... How would you advise the OP if he wants this goni to thrive in his tank?