In my experience they usually either:
(a) look puzzled because they’ve never even heard of gnosticism before;
or
(b) reply something like “No, Gnosticism is about Nature (or human sexuality) being created evil and needing to be destroyed, or about people doing anything they want (including sexually) because the body doesn’t matter, or Christ being a lesser goddy-type thing, or Christ never being Incarnated, or all things being God but ignorantly so and having to be wakened by the Christ messenger, or YHWH being basically Satan” (which are indeed various beliefs reported of Gnostic groups, but none of which have anything remotely to do with why the groups were called Gnostic).
A few people have read enough to get it, so they either already know it when I mention it, or they put the pieces together fast (in which case they’re either amused if they already have problems with the concept of salvation by doctrinal knowledge, or else they’re stunned and troubled that they’ve never figured out the connection before.)