Board Thread:Questions and Answers/@comment-28292244-20160424121523/@comment-27701762-20160424191310

HighOnTemFlakes wrote: I guess, but none other monsters have special eyes that glow other than Asgore. They might have learnt it from somewhere? Or it could be an external representation of powerful magic. Or it could be a result of Sans's experiments with the timelines (which he may or may not have been doing with Gaster). Or it could be a unique ability that Sans and Asgore have, since several monsters have unique abilities (Undyne's spears, for instance).

And the eye itself isn't exactly proof of anything. Note that the eye doesn't flash with every single attack Sans makes (even with the Gaster Blasters), which is what we would expect if it is connected with his ability to use magic.

Which doesn't mean that it is impossible that Sans learned anything from Gaster. Perhaps the Gaster Blasters were imparted to Sans in some way by Gaster, who in turn shared them with his brother. But this would be a significantly different conclusion from the initial claim that Sans learned how to use magic in general from Gaster.