I started a game in Dev and looked for the option you referred to but couldn't find it. Where is it?
You need to press Ctrl+shift+. to bring up the menu. This article has a rough tutorial on its features. To get the ones you are looking for, from the main menu go to change galaxy, then change player. The first two options are change current player, which allows you to change which player you control, and disable AI, which will prevent an AI player from giving any commands (you can do it again to enable it). Note basic unit AI behavior like auto attacking things in the gravity well won't be disabled by this, though you can just manually switch control and disable auto attack manually.
You may want to mess with the other features like spawn entity as well, which can allow to spawn whatever ship you like at the selected gravity well, which saves you the trouble of having to wait to build say a dozen Kol battleships to test GRG.