In the Monster Manual, gargoyles, golems, helmed horrors, and xorns all enjoy either immunity or resistance to:
bludgeoning, piercing, and slashing damage from nonmagical weapons that aren't adamantine
Is it actually possible to obtain a nonmagical adamantine weapon?
- Although the Dungeon Master's Guide describes adamantine armor (p.150) and Daern's instant fortress (p.160) as being made of this metal, and provides an AC for destructible adamantine objects (p.246), there's no mention of adamantine weapons.
- The creation spell (PH p.229) can produce an adamantine weapon that lasts for a single minute.
- The true polymorph spell (PH p.283) can't turn a regular weapon into adamantine, but it might be able to turn a goblin into one for an hour.
- The only way to obtain a (somewhat) permanent adamantine weapon in the game seems to be to loot one from drow elves (MM p.127).
- A PC (perhaps even a drow) with proficiency in smith's tools could presumably craft a nonmagical adamantine sword during downtime at 5gp per day (PH p.187), except that unlike silvered weapons, there's no pricing for adamantine.
It's seeming like mugging an NPC drow is the only by-the-book way to tool up against gargoyles.