Portage on top of Fuchsia, probably. There's not all that much more to Gentoo all things considered.

Zircon is miles apart from linux. It’s like saying you’d run Portage on top of Windows. You and what army?

Windows was built with a POSIX subsystem from the very beginning. WSL 1 uses something analogous.

https://en.m.wikipedia.org/wiki/Microsoft_POSIX_subsystem

The Zircon kernel does not support signals, so basic C is not going to work well.

"It is heavily inspired by Unix kernels, but differs greatly. For example, it does not support Unix-like signals, but incorporates event-driven programming and the observer pattern."

https://en.m.wikipedia.org/wiki/Fuchsia_(operating_system)#K...

As long as there's Bash and Python support, Gentoo Prefix could as well. That's kind of the only things that are hard needed for Gentoo as Portage is written in Python and Ebuilds are Bash scripts. Bigger issue would be that the Fuchsia kernel likely isn't POSIX complete.

Gentoo Portage had support for running on top of Interix, so it did run on Windows at some point (over 15 years ago).