Hello Aysun, It is currently not possible to add a GPU to Google App Engine Flex. You may be referring to adding a GPU on a Virtual instance[1].
[1] https://cloud.google.com/gpu/ -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/03900cb4-469f-46d7-8f12-02d48b99ec0b%40googlegroups.com.
