Gingerbread Roof Outer Corners were implemented during the May 2017 launch update, together with the other Roof and Slope Corner blocks.
Gingerbread Roof blocks and recipes can be found in randomly spawning Holiday Gift-boxes during the Festive Season event around Christmas. There are also player-created adventures that offer these recipes.
Outer Corners are created by putting Gingerbread Roof blocks into the processor, giving you 2 outer corner blocks per roof block.
The roof blocks can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.