Some characters might be amused. For example, Sans would probably make some witty remarks about the fanfiction's plot twists. Papyrus, on the other hand, might be a bit confused at first but then get really excited if he finds a heroic role for himself in the fanfiction.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of Undertale fanfictions, and you might be able to find Gaster x Asgore yaoi ones by using the search function.
There might be a fanfiction where Gaster, with his mysterious past and knowledge, helps Asgore deal with the guilt of past actions. As they spend time together, they discover new sides of each other and a yaoi - themed relationship blossoms. It could involve Gaster showing Asgore different perspectives on leadership and forgiveness.
Frisk could be one of the characters watching. Given their role in the game, they would be interested in Sans' fight as it could impact their journey through the Underground. Additionally, Undyne might be present. She's a strong fighter herself and would probably be sizing up Sans' techniques, maybe even learning from him or being critical of his fighting style depending on her mood.
Sakura could be involved. She could be like a healer or an advisor in the new world. Her medical skills would be very useful. Also, Sasuke might be there. He could be a sort of enforcer, using his Sharingan to detect threats. In the world of Asgore, they could form a new kind of team, with Naruto as the leader like Asgore. And then there could be characters from Asgore's world like Toriel. She could be a guide to Naruto about the culture and history of the place.
A 'asgore depressed fanfic' might explore the character Asgore's state of depression in depth. It could show his internal struggles, perhaps related to his past actions or the weight of his responsibilities. The fic may delve into his feelings of loneliness or regret.
King Asgore is a character from the game Undertale. He is the king of the monsters in the Underground. He is often depicted as a large, goat - like creature. His story is one of loss and hope. He lost his two children and was left to rule his people. He is in a difficult position as he has to deal with the situation of his people being trapped underground. He also has to face the possibility of war with the humans on the surface.
One interesting thing could be the different reactions of the characters. For example, some might be in awe of Sans' combat skills, while others could be worried about the outcome. They might cheer him on or try to interfere depending on their relationships with Sans.
The Weasley twins would likely make light of the situation at first. They might crack some jokes to ease the tension. But they would also use their own experiences to tell Harry and Ron that fighting over small things isn't worth it and that they should patch things up quickly.