Is CJ in GTA 6?
The direct answer
No. CJ is from GTA: San Andreas’ 1990s Los Santos — a different era and universe — and isn’t part of GTA 6.
Carl "CJ" Johnson belongs to the earlier 3D-universe San Andreas (2004), separate from the modern HD-universe timeline GTA 6 sits in. Rockstar has given no indication he appears.
GTA 6 is set in present-day Leonida with new leads. A San Andreas-era character returning would break the series’ continuity, and nothing suggests it.