They are two completely different projects with completely different goals. Indeed Go was started by Rob Pike, Ken Thompson and Robert Griesemer, the first two

They are two completely different projects with completely different goals. Indeed Go was started by Rob Pike, Ken Thompson and Robert Griesemer, the first two of which also worked on Plan 9, and the Go compilers can trace their lineage back to the Plan 9 C compilers (with some pit-stops between there). Plan 9 was designed with the retrospective on UNIX and it's trappings, with a focus on networked distributed systems. Fuchsia is by a completely different team not involving Plan 9 alumni (as far as a I know) and has a different target, mostly aimed at embedded systems. If I recall correctly fuchsia development involved some alumni from the BeOS/Haiku world but don't quote me on that.