abraxas

Game Mastering is not just one job and different games handle or overlook the different GM hats in different ways. The Angry GM has a nice way of breaking what is considered the GMs job into:

Running the game. The actual at the table game. How to narrate, make rulings and describe results. Game systems with easy to adjudicate rules are going to be easier on the GM. In my experience simple systems like PDQ are very easy to run. Player facing mechanics some times also help ease the cognitive load of running the game like Cypher system or Symbaroum. System familiarity plays a huge part here.
Scenario design. Writing and preparing for the game. Very few games actually explain how to build scenarios appropriate for them in a clear structured way having the GMs mostly on their own. The best resources I've found are the new "So You Want to Be a Game Master" from Justin Alexander and the various books from Kevin Crawford. Complex systems and systems that expect a tight balance between PC and their opposition tend to make this harder.
Campaign management. Managing the game and its players. Game scheduling and keeping the game running. Resolving conflicts between the players, etc. There are very few resources for a GM on how to do these. Part project management, part event organizer most of the advice for the GM is found in reddit comments and forum threads.

The Angry may have a rambling introduction for each of his articles but he has touched on many of these issues and I've found his game advice invaluable.