At first, Sans would probably be a bit shocked, but then he'd turn on his laid - back, sarcastic attitude. He might make jokes at Gaster's expense, which could either annoy Gaster or make him respect Sans more. If Gaster tries to harm him, Sans could use his shortcuts to dodge attacks and try to find a way to reverse the situation. He'd also likely be thinking about how this affects his friends in the Underground, especially Papyrus, and try to find a way back to them.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
Often, they interact with a lot of patience. They sense the reader's pain and are gentle. For example, they might start by just being a quiet presence around the reader. They slowly build trust, like in a story where a Sans from an AU offers the reader a small gift as a sign of friendship without pushing too hard.
Papyrus might be initially shocked. He's used to Sans being more laid - back, so seeing him in a fight could be quite a surprise. His eyes would widen and he might gasp.