I just ran the same one-liner command, and I do see the same key ID printed and yet the installation passed that point in question. So I assume either something has been fixed since OP's post, or it was just a temporary glitch.
root@f5d45d77b415:/# curl -sSL http://get.gazebosim.org | sh
=======================================================
GAZEBO INSTALLATION SCRIPT
=======================================================
This script is installing the latest stable version of
Gazebo Simulator available from your package manager
In Debian this script will setup the osrfoundation
repository to install the latest package available
+ sh -c apt-key adv --keyserver keyserver.ubuntu.com --recv-keys D2486D2DD83DB69272AFE98867170598AF249743
Executing: /tmp/apt-key-gpghome.ziAagWXNKe/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys D2486D2DD83DB69272AFE98867170598AF249743
gpg: key 67170598AF249743: "OSRF Repository (OSRF Repository GPG key) <osrfbuild@osrfoundation.org>" not changed
gpg: Total number processed: 1
gpg: unchanged: 1
+ sh -c mkdir -p /etc/apt/sources.list.d
+ sh -c echo deb http://packages.osrfoundation.org/gazebo/ubuntu\-stable bionic main > /etc/apt/sources.list.d/gazebo-stable.list
+ sh -c sleep 3; apt-get update; apt-get install -y -q libgazebo11-dev gazebo11
Hit:1 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
:
Have you tried also the step by step installation?