"What kind of grandmother sells a gift given to her?"
Uh, this lady would, have you met her? She's all about the souls. When she mentions that you need to get the soapstone so you can be summoned to other worlds, she starts giggling at how many souls you can bring her. She would straight up sell her own grandkid if she could.
In her dialogue on the soapstone, she does refer to the bit of "soppy gossip" that is "cordial intrusion layeth the path to embers," which is exactly what Sirris tells you. Interestingly, "gossip" can refer to the piece of information itself, or in archaic English (like the kind the handmaid uses), she could be referring derisively to Sirris herself. Slow down with those burns, handmaid.