Salam Claudia,
Actually since you asked about "God forbid" and its relation with "قدّر الله", it means exactly the opposite!
God Forbid: God is not allowing something to happen.
قدّر الله: Allah/ God is allowing something to happen.
To make it equal to God forbid, you should add لا to it, like this: لا قدّر الله.
Let's say you want to use it with sickness and war, here how it's possible in this situation:
1. لو اشتعلت الحرب في البلاد لا قدّر الله، سنغادر إلى دولة أخرى:
If war started in our country "God forbid", we would leave to another country.
2. لو أظهرت التحاليل الطبية إصابة الفتاة بالمرض لا قدّر الله، سأقنعها بأن تبدأ العلاج فورًا:
If medical tests showed that the girl is sick "God forbid", I would convince her to start treatment right away.
Now if you want to use the opposite (قدّر الله/ God Allow) in those situations, look at these 2 sentences:
1. قدّر الله أننا كنا خارج البلاد عندما اشتعلت الحرب:
Allah/ God allowed that we were out of the country when the war started.
2. قدّر الله أن تحاليل الفتاة الطبية أظهرت خلوها من أي مرض:
Allah/ God allowed that the girl's medical tests showed that she's not sick at all.
I hope this helps :)