Gboard's autocorrect is not available in German
When adding a new language to Gboard for the first time, it may show a notification "Updating your typing experience", possibly for downloading/updating the relevant dictionary.

If it didn't happen, try removing and re-adding the problematic language's keyboards.

In the worst case, you may need to reset Gboard's data by clearing their app's data (generally from the phone's Settings - Apps)