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Can't work out what object use for casting

The blueprint I am trying to cast to is my AI character.

The Object pin needs an Object fed into it. Right-click and use a “Get All Actors of Class” node. Select your AI Character blueprint on the drop-down. Connect it to the Object pin.

Hamasam that totally depends where that begin play event is.

If you have just one Gnome in level you can try “get all actors of class”, set it to gnome, then get function to get element 0 from result array.
Buf for better answer you should post more information about your project (more pictures).

Theres also the “cast to class” option. That may be what you need in this case