Dear @Husein_91,
Welcome to the Wefugees Community and thank you for reaching out to us.
I am not a lawyer but based on the information you provide here, I agree with the legal consultant. You did good in contacting a local consultant as they usually know the local authorities and proceedings best. I don't believe that the immigration office (Ausländerbehörde) is eager to deport you (which is at the end quite an effort and associated with high financial costs) if you are actually willing to leave Germany voluntarily.
The The so-called "Grenzübertrittsbescheinigung" (GüB) is a document that you have to submit to officials at the German border who will then inform the immigration office that you left Germany.
I hope this helps and please don't hesitate to get back to us to keep us updated or with any further questions.
All the best,
Meike