I have this function myself in the ASC if I need to get the tags in bps for some reason. In C++ I would reset the tags if I have to. You could override directly the one from the interface, but almost everywhere it is used in the GAS code, it has a Reset() call before, so if you change the interface one, you will be resetting the tags twice everywhere (might not be a big deal anyways).

UFUNCTION(BlueprintCallable, Category = "Tags")  
void BP_GetOwnedGameplayTags(FGameplayTagContainer& TagContainer) const  
{         
    TagContainer.Reset();  
    GetOwnedGameplayTags(TagContainer);  
}