Gaster is on his way to grillbys when he senses a spike of pain then the almost absolute lack of response. He heads into the forest and when he is just out of sight of the town he finds the skeleton he met and has been watched by for a while. The skeleton is cold and looks like they have a broken ulna. That must have been the spike of pain he felt from the soul. He looks up and sees what might have been the branch he fell off of cause breaks like this don't happen very easily.
He picks him up and carries him home. He checks his stats and his hp is almost 0.1 which is extremely low. And considering the fact that his hp was very low to begin with, this is bad. Thankfully he is already passed out when he sets this skeletons ulna.
Sans wakes up in an unfamiliar place, until he realizes that he is in the guest room of his childhood home.