Having some full set gasket kit of Subaru Impreza EJ204 in stock, when a customer asking for the Subaru WRX STI EJ257 full set gasket kit, I try to figure out what should be replace in the EJ204 gasket set content, in the first photo, the 2 pieces of cylinder head gasket need to be change, the 2 pieces of main intake manifold gasket and the waterpump gasket is same, and there are 2 pieces extra surge manifold gasket to put in, plus a piece of different type of throttle body gasket.
The exhaust manifold gasket, exhaust pipe gasket, timing cover gasket is basically same for both EJ204 and EJ257.
In the above photo, the rocker cover gasket need to be change, and you may take out the 16 pieces of rocker cover bush and 4 pieces of half moon seal from EJ204 as the EJ257 type of design didn't use those item, other o-ring items is basically same for both model, but the 4 pieces of gasket in the bottom right of the above photo is not in EJ204, until here, I just lack of this 4 pieces of gasket.
Photo in bottom showing the flywheel oil seal which is same size for both engine, of course it just need one piece for an engine, below photo is just for illustrated purpose.