This error doesn't block the users to go into Play Mode, but causes Build and Run to fail for the project, every time.
If you look at all the assets in the scene hierarchy, there is no GUI Layer.
Also, if you delete the library and import the project again, the error goes away.

To reproduce this issue:
- Download this project
- open it with Unity 2019.3.0b1
- Open the TrailerText Scene
- notice the error and warning

Error message:
Component of type GUI Layer is no longer available in Unity. References to it will be removed!
Component could not be loaded when loading game object. Cleaning up!

Reproducible in project upgrades from 2017.4 to 2019.3 and from 2018.4 to 2019.3
Cannot reproduce if the Asset Database is set to Version 1 (but that also might be due to the library refresh)
Reproducible with Unity 2019.3.0a4, 2019.3.0b1, 2020.1.0a2
Cannot reproduce with Unity 2019.3.0a2, 2019.3.0a3

Workaround: deleting the library and re importing the project again will make the Error go away.