By referring to Marlow as being a part of a 'new gang of virtue', he is connecting Marlow with Kurtz. The 'new gang of virtue' also refers to a new type of European sent out to re-analyse colonization. They are not missionaries in the religious sense but are sent to take another look at the natives and the system in general. That is why the Brickmaker lumps Kurtz and Marlow together as being two such men.
The Brickmaker is suspicious of Marlow and thinks he is lying through out their whole conversation. He also indicates with this statement that the same people who sent Kurtz out to Africa in the first place, have now sent Marlow in to remove Kurtz. The reader knows this is not the case, however it is what the Brickmaker thinks. Marlow also does not do anything to clear up the Brickmaker's impression of him. This connection between the two is essential to the plot as it helps the reader understand that Marlow himself not only identifies with Kurtz but others also saw 'Kurtz' in Marlow.