gdm3 has an annoying bug for Nvidia Optimus users not using wayland. If you run your nvidia driver kernel modeset (modeset=1), you enable "prime sync" which means you no longer have tearing on your laptop screen. prime sync only works with modeset=1, so most Optimus users will want to enable this, unless they don't use their laptop's display, or don't care about tearing.
In Ubuntu 18.10, the nvidia packages activated modeset=1 by default, and the upgrade to 19.04 preserves this setting.
19.04 (new install) does not activate modeset=1, to avoid a big gdm3 bug.
Problem: gdm3 kills the nvidia card from displaying to external monitors when your use modeset=1. Bug still exists in Ubuntu 19.04. Speculation is that gdm3 sees the driver in modesetting operation, and decides wayland must be in use, and there is no way to get it to behave otherwise. It must be a hard bug to fix.
lightdm does not have this problem (sddm also does not have this problem). So this is a very good reason for Ubuntu users to swap gdm3 for lightdm.