Effect only activates when sent to the graveyard via card effect.
Gem-Knight Fusion Monsters
Use Gem-Knight Lazuli as a fusion material when fusion summoning your Gem-Knight fusion monster. This counts as sending Gem-Knight Lazuli to the graveyard via card effect allowing you to retrieve a Normal monster.
The original Magnet Warrior monsters are all normal monsters which can be retrieved by Gem-Knight Lazuli. She also synergizes well into Magnet Warrior themed decks as she herself is an EARTH Rock-type monster.