what gender is Ammy?

Female, there has been swali about that, but she is, the God's refer to her as the Great Mother Amaterasu, so obviously she must be female and the fact that her apparent child Chibiterasu appears in Okamiden, also suggests that she's female.

I totally agree. She's a girl.

I know she is female but why was she panting at the size of roa's breasts, was she jealouse because of the size of all six of hers?

Maybe Ammy's mbwa mwitu form is male, but her spirit is female?
That may explain the reaction to Rei's breasts.

In the game she is referred to as female but the manual states that in Japanese mythology the gods have no gender, so any answer is correct.

It a she