Hi and1lea!
Thanks for contacting us with your question!
Yes, it is possible to use flag series from the Highcharts Stock module in Highcharts Gantt. You should import Highcharts Stock as a module and Highcharts Gantt as usually. Actually, there are some small bugs, like creating additional yAxis rows for flags, but you can hide them using
yAxis.breaks property. Please, check the demo below to see an example.
Demo: https://jsfiddle.net/BlackLabel/ze7odjbL/
API Reference: https://api.highcharts.com/highcharts/yAxis.breaks
Let me know if you have any further questions.
Regards!