Destroy target permanent. Its controller creates a 3/3 green Elephant creature token.
Generous Gift is white's premier catch-all removal, and the reason it's in thousands of decks is flexibility. For three mana at instant speed, you can destroy any permanent — creature, artifact, enchantment, planeswalker, or even a problematic land. That kind of unconditional answer is rare in white, a color historically starved for flexible interaction.
The downside — giving your opponent a 3/3 Elephant — is almost always trivial. You're trading a 3-mana instant to blow up a game-warping commander, an Avacyn, a Cyclonic Rift-protecting artifact, or a key combo piece. A 3/3 is a small price to neutralize a real threat.
It belongs in essentially any white deck, from casual to cEDH. It's especially valuable in decks lacking other colors' removal suites (mono-white, Boros, Selesnya).
When not to play it: highly aggressive token or aristocrats decks where you're racing, and giving an opponent a blocker matters — or if you have access to cleaner removal like Swords to Plowshares for creatures. Even then, the versatility usually wins.
If the target permanent is an illegal target by the time Generous Gift tries to resolve, the spell doesn't resolve. No player creates an Elephant. If the target is legal but not destroyed (most likely because it has indestructible), its controller does create an Elephant.