I would like to highlight 2 weeks as holiday time in my Gantt Chart.
It should look like the typical christmas break that is visual as a grey column.

But:

I do not want the time to be defined as time off. The highlighted weeks should be considered as working hours.

How can I set that up?

Thank you in advance.

Take a copy of your Standard calendar and add your 2-week period to this "copy of Standard" calendar using Exceptions.

Then go to your Gantt chart Timeline and change the calendar for Now-working time to "copy of Standard".

Your Tasks will have the Standard calendar while your Gantt chart will use the "copy of Standard" calendar with the only difference between the 2 calendars being the 2-week period you want to highlight.