I found out a solution to this problem by studying gnome-control-center source code. it looks for an application which has support for "text/calendar" mime:
https://github.com/GNOME/gnome-control-center/blob/master/panels/info/cc-info-default-apps-panel.c#L167 the solution is as following - from the command line, enter the following command gio mime text/calendar org.gnome.Calendar.desktop voila! you can now see gnome-calendar in the control center and it is the default one no idea if this matters, but I did it as regular user (non-root, no sudo) -- pozdrawiam serdecznie, Mateusz Mikołajczyk, a.k.a. toudi
