Remove the black stuff off the new gasket?
180 and 126 on that pic are two different gaskets. 180 should be a big square rubber o-ring. that one you just pull the old one, and stuff the new one into the groove of the elbow. if you also bought the 126 gaskets as plan to use it or you just plan to reuse the old 126 gaskets on yours currently, i would suggest removing the black film on the metal 126 gaskets and coating it with RTV just prior to install. i cant remember if the original gasket of mine had the black coating, thats why i stated if it had it, remove it.
i had the o-ring fold over on me on the original install and leak. when i removed it, i put a new metal 126 gasket in and the black coating was causing it to not seal, so again i had to pull it all apart, cleaned that crap off with a scotch bright wheel and used RTV on both sides. been solid for 2.5 years now.