I can tell you the only rolling stock on which the trucks can pivot far enough would be reefers and box cars. On all the others the couplers will hit the steps on the bodies first. Gilbert two rail flanges are much smaller than Lionel wheel flanges and are designed for the Gilbert flat top, prototype profile rail. I suspect a cut of cars may stringline when attempting to pull them around an 027 curve on Lionel round top rail.
An Atlantic engine may go around an O27 curve without derailing. Keep in mind all the Gilbert trains are full 1/64th scale length, the passenger cars just model 70' cars. Even when these 1/64th scale trains were introduced by Gilbert pre war in O gauge they were not designed to run on O27 or O31 track. Gilbert prewar O gauge three rail track was 40" in diameter.
The best way to test smaller radii would be to get a couple pieces of S gauge GarGraves flex to bend to small test curves.
