Not sure if it is necessary, but thinking about what would fall under gradient eyes versus partial heterochromia, I think you have somewhat two different but similar concepts.
Partial Heterochromia should require that there are at least two distinct eye colors in the iris, but gradient eyes doesn't necessarily mean that the iris is two colors.. it could mean that the eye color is in a gradient which could be more due to lighting or a representation of the condition the character is in (depressed, moody, etc).
On partial heterochromia, I think that needs to have it's own tag, if there are imagines needing it. I have nothing against it also going under heterochromia, just that complete heterochromia is so much more represented that to be able to find partial heterochromia you'd require a specific tag.