Getting enrolled in gymnasium

There are various entry requirements you need to meet to be allowed to enter "Gymnasium" (the highest kind of secondary school in Germany, leading to university entry eligibility).

One of them is German language skills at a level sufficient to follow classes - do you have that?

Another one is knwledge in all subjects at the level of the class you want to enter. There might be an entry test for those.

Foreigners coming here with less than fluent German and lacks in other subjects are usually sent to separate preparatory classes before they are allowed to join the normal ones (which usually means losing a year, academically). Alternatively, they could tell you to enter one of the lower ranks of secondary school instead - "Realschule" or "Hauptschule".

You should contact the school authority ("Schulamt") of the town you plan to move to!