: The Stellaris Wiki is an excellent resource for up-to-date information on game mechanics, including console commands. You can search for "megastructure" or "gigastructural engineering" topics.
If you just want to test things out without worrying about mineral costs or influence: stellaris gigastructural engineering console commands link
Since Gigastructural Engineering has hundreds of assets, the easiest way to find an ID is in-game: : The Stellaris Wiki is an excellent resource