Does women's asking about the news and condition of a woman who is absent from their gathering, and the reason for her absence, count as backbiting (ghībah) or slander (namīmah)?

Backbiting is your mention of your brother with what he dislikes of the flaws that are present in him. However, if they are not in him, then it is slander. Tale-bearing (Namimah) is conveying speech between two parties to cause mischief between them, whether what is conveyed is true or false, and whether the intention was to cause mischief or not. The consideration is for what the matter will lead to. Both are forbidden by Islamic law. As for one of you asking about her friend, if it does not involve describing her with what she dislikes, or lead to mischief, then it is neither backbiting nor tale-bearing.