Gemini is deniable from 100% HP in ultimate form.

[MAXBobbels]

I assume it works with every dot, but I only tested slither's spray and his passive.

When he has a dot you can deny him no matter how much HP he has.

[Inomares]

Can confirm this. Seems to work with all normally valid DoTs.

[ElementUser]

This is a hard-coded issue, not fixable through scripting