Most GameMode classes cannot be set in Blueprints. Only Set Default Pawn Class and Set HUD Class have Setters available in Blueprints (the Getters are all still available).
Reproduced in 4.8.0 and Main (Most GameMode classes cannot be set in Blueprints). This is a regression: all GameMode classes were available as Setters in 4.7.6.
1. Open any project
2. Open the Level Blueprint
3. Add a Get GameMode node to the Event Graph
4. Drag off the Get GameMode node and search for "classes"
In the Variables > Classes section of the menu, only the Set Default Pawn Class and Set HUD Class Setters are available. In 4.7.6, all of the GameMode variables were accessible as Setters.
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