I learnt that the Rabbanim don’t make a geder on a geder so why is eating milk not allowed imedietly after eating chicken, since chicken is not allowed to be eaten with meat because of a geder and waiting 6 hours is a geder which makes this a geder on a geder?

The rule about not making a gezerah upon another gezerah is not an iron clad rule. There are many cases where we do make a gezerah upon another. The Tosafot in Shabbat 21a explains that when something is prohibited in and of itself even though the prohibition is Rabinical we can apply another gezerah upon it.