I assume rockstar didn't pay, so they leaked the source code. Not sure why anything would be delayed because of this.
Why would they pay? It's a 10-year-old game that's the second best selling game of all time. Rockstar made their money, and there isn't anything a competitor could use to gain an advantage. It's almost good because it's free press for GTA VI.
2. GTA online brings at least half a billion a year in microtransactions.