Given the time constraint, that's understandable.
If you're still struggling with implementing that, maybe I can help a little by giving several suggestions. I think you can try and create a timer that starts counting down when the game starts, and once that timer reaches 0, you make the ghosts appear. You can make several timers, one for each ghost, and maybe you can set different time for those timers.
I'm not sure how exactly do you implement those in GDevelop, but I believe you can do it ^^