rosdep can't resolve keys for [rbdl] and [rrbot.gazebo]

[tckarenchiang]

Hi,
I am using ROS melodic on Ubuntu 18.04.

When I tried to install all the dependencies of the gravity_compensation_controller_tutorial pkg listed in the package.xml, I got the following errors:

ERROR: the following packages/stacks could not have their rosdep keys resolved
to system dependencies:
gravity_compensation_controller_tutorial: Cannot locate rosdep definition for [rrbot_gazebo]
gravity_compensation_controller_tutorial: Cannot locate rosdep definition for [rbdl]