A 3-mana mono-red legend from The Hobbit that grows and pings the table off every noncreature spell, plus its own Adventure half — a card-selection engine wrapped around a spellslinger threat.
Spellslinger decks. Every noncreature spell cast grows Gandalf and pings the table, rewarding a deck built around casting a lot of instants and sorceries.
Wizard tribal. The Adventure side's exiled cards can only be played by a deck with a Wizard on the battlefield, making it a direct payoff for the creature type.
Aggressive red decks. The combination of a growing body and repeated table-wide pings is real reach on top of whatever else the deck is doing.
How it does in Commander
As the commander
As the commander, Gandalf rewards a mono-red or Wizard-tribal spellslinger build directly, since guaranteed access to both the noncreature-spell trigger and the Adventure side's card advantage from the command zone supports a deck leaning fully into both halves.
In the 99
A solid include in any red spells-matter or Wizard-tribal deck even without leading it, doing real work as both a growing threat and a source of card advantage.
Formats it's strong in
Wizard tribal decks. The Adventure side's card advantage is a direct payoff for the creature type, rewarding a deck built around it specifically.
Spellslinger and prowess decks. The noncreature-spell trigger fires off cards those decks already want to run, adding reach and growth on top of their existing plan.
Formats it's weak in
Creature-heavy decks. With few noncreature spells to cast, the triggered ability has little to work with.
Decks with no Wizards. Without a Wizard on the battlefield, the cards exiled by the Adventure side can never be played, wasting that half of the card entirely.
Synergistic cards
Young Pyromancer — Another noncreature-spell payoff that stacks its own token generation with Gandalf's growth and pings.
Guttersnipe — A direct-damage payoff for casting instants and sorceries, doubling up on the reach Gandalf already provides.
Any cheap Wizard — Getting a Wizard onto the battlefield is what unlocks the Adventure side's exiled cards, so even a small one is enough.
Lightning Bolt — A cheap, efficient noncreature spell that triggers Gandalf's ability for minimal mana investment.
Is it worth the price?
As a recent-set card, Gandalf, Goblins' Bane is affordable and a reasonable pickup for a red spellslinger or Wizard-tribal deck — low financial risk for a genuinely flexible card.