There is a Gnome calendar app: https://wiki.gnome.org/Apps/Calendar.

gurkendoktor · on July 22, 2017

And then there's also https://wiki.gnome.org/Apps/California

And of course elementaryOS has its own GTK+3-based calendar: https://launchpad.net/maya

So basically, the same situation as with mail clients, photo apps, music players, ... I wish the GNOME world would decide to just merge all the features into one app per task and then kill the others.