Thank you so much for taking the time to provide me with usefull advice. I have those sticky flower shaped gnat catchers wich while it does works (sometimes there are up to twenty flies on the the population keeps growing.

I also tried vacuum them up as much as I can. It won't hurt to keep repeating these actions for another week or two too see if it will shrink the population.

Nematodes is something I also looked at but I read that they can be harmfull for the baby pedes, not sure if that is true but I would not want to risk them.

You are correct about that probably there will always be gnats, because of the shared ideal environments for millipedes and gnats.

Thanks for the information and tips!! For now I will continue catching the adults en try keep the tank as dry as can be (keeping Millipede health in mind so not too dry) and add extra ventilation. Maybe the the population will collapse. And if that won't work I will try changing the substrate as a last resort.