Just wondering, Zune is MUI based.
Only if by "MUI based" you mean that it's a MUI reimplementation

MUI recently had an updated. AROS and the MOS teams have been known to exchange code, or help out in areas...
Would i be wrong in assuming that both teams or all three teams (MOS, AROS, MUI) helped out each other or something similar to get the gradients working on these platforms?
Yup, you'd be really wrong

We don't get MOS' own proprietary code, we only get bug fixes to the code
WE give them, and sometimes also some new code, but which was already open source.
--- EDITED ---
CISC, of the MOS fame, just made me notice that they've also contributed back their packet implementation (DOS library stuff). However, that implementation has not made it to the AROS source tree and probably never will, for reasons related to design decisions
--- /EDITED ---
I've implemented gradients in Zune all by myself, 2 days ago.