Both Gifted and Test subject consist under similar timelines, but diffrent choices changed the outcome of both stories 

Since Gaster did visit Asgore a few days previous of the beginning of test subject and the two bound there souls Sans and Papyrus would of been produced anyways  leading to well... Gifted My other story about timelines 

Since Gaster was deep in his work (test subject) Sans and Papyrus still appeared in the timeline but through other means 

thought i might mess with your heads a little bit while you wait for the next part ;3