/vr/ - Retro Games

Question for oldfags. Wikipedia states that id released the Doom's "game source on December 23, 1997, placed under GPL on October 3, 1999.".

But what was the deal at the time from a user/wannabe modder point of view? So you could download and read the entire source code, and then? Have you some kind of SDK/editor to create maps or these came separately?

Also genuilely curious to know if Id were truly the first to have released their work under free license.