1. If gheebah was done on a kafir, straight up atheist, is it obligatory and/or mustahhab to ask for forgiveness?

2. If gheebah was done on an Ahlul Kitab, is it obligatory and/or mustahhab to ask for forgiveness?

3. And is gheebah on nawasib halal?

by

1 & 2 Sayyid Sistani says you should avoid doing so and it is good if you ask for forgiveness

Gheebah is not halal on anyone, speaking about anyone only becomes permissible when the conditions are met (e.g. they are oppressors, openly fasiq, or harming others)