You remember that shit Sans does at the end of his boss fight
Where every time you try to move to the left side of the box, he flashes his eye and teleports you back to the center?
https://youtu.be/Vr4IYjeplJA?t=617

Gaster is gonna pull that shit on you in a dialogue choice, railroading you into picking the option he wants you to pick, and teleporting you back to the center whenever you try to pick the other option