Special summons a low-leveled normal monster from your graveyard.
Revival effect only activates when he is sent from hand to graveyard.
Some cards have the cost of sending 1 card from hand to the graveyard to activate their effect. You can send Gem-Knight Obsidian to the graveyard and then revive a normal monster to recover your loss. Note that “send from hand to graveyard” is different from discarding.
One way you will be sending Gem-Knight Obsidian from hand to graveyard is by using him as a fusion material. Here are the spell/trap cards you can use to send him from hand to grave which you can use in a Gem-Knight deck.