I want to make sure this is high in search results. As eXi mentions above, you can select the game mode via the options string with Game Mode Class Aliases set in the Default Modes section of Maps & Modes in Project Settings. Be sure to click the down arrow to expand the section. Different game modes in the same level for UE4. Multiple game modes for the same level.
Although he’s mentioning the OpenLevel URL, this is also possible with the options string in the OpenLevel node, which might be the same thing.