I installed 'server' to begin with and upon reboot, no X-Windows started. I just got a command line login prompt. After I logged in, startx didn't work either.

When I did the default install without chosing any option, it installed GUI just fine and started it.

So what are the differences between server and workstation options and how does one get either of them to start GUI upon boot (Gnome)?

Since you've been asking about RHEL for a while now, did you look at the Red Hat knowledgebase? Are you PAYING for RHEL?? Because if not, CentOS is 99.x% identical.
https://access.redhat.com/discussions/3772571
https://access.redhat.com/articles/911363

There are also MANY Red Hat knowledgebase articles about how to install the X/Gnome environment.
https://access.redhat.com/solutions/4479561

Server doesn't include the GUI unless you select it; workstation does, obviously. There are other differences as well, all listed on Red Hat's website. Read the "Question Guidelines" link in my posting signature, and do basic research first; after SIXTEEN YEARS, you should have a good starting point by now.

By default, Server has no GUI, Workstation does.

To install the GUI on Server and make the GUI the default target:
To be precise, I only tried that on Centos 8.