well, it's now been a while since you start helping your new friends on defeating Betty, you're still surprised that everyone forgave you from your mistakes that made Betty kill sans and handing her the Hate vial by mistake, well...not everyone did forgave you, Gaster still mad and furious about this, which is understandable for you since he literally lost his son because of you and you accidentally made Betty stronger by the Hate...oops. But now after he got a talk to talk with papyrus about forgiveness,he start thinking about it more, since you're really sorry, but do you really deserve his trust now? Only time would tell

After you got a call from the chef leader of the AMD (Ronan) to meet with him at the city arena to talk with Rave to discuss some plans in order to keep the citizens safe, but you couldn't go on alone cause it's way too dangerous by all the kumulings roaming around determino, so you have to at least take someone with you for protection, and as you didn't expect, Gaster volunteers to come with you while papyrus watch over sans at home. Great. Now you have to stay with him and hope things won't get ugly between you two now that you're in the same Van

Gaster:he stayed silent while the Van is moving, he couldn't make an eye contact with you since he felt so awkward remembering what papyrus told him about forgiveness