This is the 11840th most frequent Turkish word.
gerçekleşmediği
"Gerçekleşmediği" means "the fact that it did not happen/come true."
In this case, 'gerçekleşmediği' is used to express impossibility or a prerequisite of realization.
Dediğiniz gibi, önerileriniz gerçekleşmediği sürece sonuçlara ulaşmak zor olacak.
As you said, it will be difficult to achieve results unless your suggestions are realized.
In this sentence, 'gerçekleşmediği' is a verbal noun form indicating that an event (the realization of the activity) did not happen.
Beklediğimiz etkinlik gerçekleşmediği için biraz hayal kırıklığına uğradık.
We were a bit disappointed because the event we were waiting for did not occur.
Here, 'gerçekleşmediği' points to a conditionality related to the realization of a project.