Explanation (written by me): Here's the low down. The Sans (AKA Ganz) was forced to take determination pills for one of Gaster's experiments (resulting in the black facial markings). Later on, Gaster died when the CORE had a meltdown, and Sans was the only one to see his death, and was therefore one of the few able to remember him. At the same time as the CORE meltdown, Undyne and Papyrus were having a training session, and by the end of it, when Undyne told Papyrus he would have to wait a few m