James Gunn can’t jump back into Guardians of the Galaxy Vol. 3 right away.
James Gunn has already written a Guardians of the Galaxy Vol. 3 script, though it is expected that he will be making some changes to it.
Guardians of the Galaxy Vol. 3 was supposed to already be shooting, and the end of Avengers: Endgame indicates that Chris Hemsworth would have been a part of the cast.
It states that Marvel and Disney are planning to have James Gunn back on set with his cast and crew sometime in 2020.