I define "metagaming" as "having your characters act with knowledge they do not possess". If I just told Michael that his character Araylia has spotted a hidden sentry in a tree, Jon cannot have his character Randal shoot the sentry (or "the empty air where a sentry might be") UNTIL Araylia passes the knowledge to Randal. Jon heard it, but can't act on it.
Inz d'Shemmar, the gnome artificer, has invented his own "collapsible steam-powered pipe organ", in order to better mesh with his adventuring group "Sparrow and the Ancients" (a group of King's Agents using the cover trope of a bard and her band touring around). One of his companions is a Wizard very interested in things that go boom, and he has a contact in the capital, a dwarf from the Blackrock Clan that experiments with explosive materials. Inz is working on creating/recreating "boompowder", based on his character knowledge of "science" (artifice), magic, and alchemy. Inz's player happens to be Randal's player... Randal does not get to invent or even think of "boompowder", but Inz because of character knowledge and gameplay is working his way there.
So Snarff's clarification of "GSP" (Gygax Skilled Play)... mostly isn't allowed at our table. Creating a character who can push the boundaries in order to invent gunpowder is really testing the edge of this "disallowance", but technically is not breaking our table's rule against "metagaming".
On the other hand... we acknowledge that our characters might have mental attributes greater than their players. As such, we allow the group to work together (when the characters are together) on ideas and planning; we don't make the player of the INT 8 fighter offer stupid ideas (or stay quiet), just as we don't expect the player of the INT 20 wizard to be a genius. The players can come up with brilliant schemes, great tactics, and draw upon their decades of game-playing. What they can't do is load up on anti-poison gear to fight the green dragon in the jungle... until they learn the dragon's attacks use poison somehow. That might be from a survivor, it might be from direct experience; it might be "remembered" from an allowed Skill Check, or learned through research. [Survivors and research sometimes are wrong... the party ended up splitting their gear between anti-poison and anti-acid, based on the information received.]