Gamble is an interesting card to say the least. It’s a card I have recently learn the power of, and when you should or shouldn’t run it. I don’t think enough decks run it, but I also definitely don’t think every deck should either. Gamble has its place in EDH, and when used correctly it can be a powerful tutor in a colour that doesn’t have many.Gamble is most useful when you don’t mind if the card you are searching for ends up in your graveyard. Gamble gives non-black edh decks access to tutor for cards they would not usually be able to search for. If you have a full hand and you are feeling lucky, by all means take a gamble. (Haha.) There are some legitimate strategies to get the most out of Gamble, as long as there is nothing in your hand you need and can’t get back. Whilst it may need to be built around a little to be most efficient, a lot of the cards that work well with it appear in most decks anyway.
If you are playing green, do not use Gamble to tutor for a creature. You have access to lots of creature tutors that are much better than gamble.
The best use of Gamble in a green deck is along side regrowth effects. If you have Gamble and Regrowth or Eternal Witness or any other regrowth effect in your hand, you can effectively tutor for anything, either for R, or for 1G and R. This can be absurdly powerful, allowing you to tutor for card draw spells or answer to your opponents threats!
Another good use for Gamble in an R/G deck is to tutor for Life From the Loam. If you end up discarding it, you can dredge to get it back. If you have Life From the Loam in hand or your graveyard, then you can use Gamble to search for the land you need, be it a cycle land to set up aggro loam, or Valakut to start burning people. You can do the same with any other dredge spell, but Life From the Loam is probably the most popular.
With Genesis in your graveyard, you can safely tutor for any creature and if you discard it, get it back in your next upkeep. Dowsing Shaman will let you gamble for any enchantment, and return it to your hand if you have to. Holistic Wisdom will let you tutor for anything and get it back as long as you have another card that shares a type with it in hand.
White has access to plenty of equipment and enchantment tutors already. I wouldn’t recommend just searching for any enchantment or equipment with no way of getting it back in case it gets discarded, but luckily, white has lots of recursion too. Sun Titan is a great recursion engine and works wonders with Gamble. You can effectively tutor for any permanent you need with 3 cmc or less. If it gets discarded, you can get it back with Sun Titan. I used Gamble and Sun Titan to great effect searching for Angelic Renewal or Goblin Bombardment as needed to complete the combo.
It also goes well with my second favourite white recursion engine...
Loyal Retainers. With Sun Titan out, you can tutor for Loyal Retainers with Gamble and Sunny T will bring him right back. If you already have the Retainers out, then feel free to tutor for any Legendary Creature. Sometimes you might even WANT to end up discarding it if it lets you get them out cheaper! With Loyal Retainers in hand you can still tutor for any legend without fear of it being gone forever if you discard it (assuming no one is ever going to play any graveyard hate ever)
Then we have Reveillark and Karmic Guide. With Karmic Guide in hand, you can tutor for any creature you need, if you end up discarding it, just get it back with Karmic Guide. With Reveillark, you just have to search for a creature with power 2 or less. If there’s a creature in your graveyard you want back and you have Reveillark, but not Karmic Guide, you can search for Karmic Guide. If it gets discarded, evoke Reveillark, get back Karmic Guide and the other creature. Otherwise just cast Karmic Guide to get it back!
White has lots of enchantment and artifact recursion, Open the Vaults and Replenish will both return things from your graveyard to the battlefield. There are also some reanimation spells such as Breath of Life, False Defeat, Proclamation of Rebirth, and many others that will allow you to tutor for cards you can easily recur.
Blue isn’t know for its recursion, though it can quite happily get back instants and sorcery's with cards like Archeomancer, or flash them back with Snapcaster Mage. There are better tutors in blue for instants and sorcery's though, in the form of Mystical Tutor, Personal Tutor and Merchant Scroll. If you have a way of recurring a spell, feel free to gamble for it though. There are loads of creatures and spells to recur instants and sorcery's, such as Mnemonic Wall or Relearn.
Body Double is an interesting blue creature. With Body Double in hand, you can gamble for any creature, and use Body Double to get a copy of it if you have to discard it.
For artifacts, you have the ever popular Academy Ruins. While a bit slow, it allows you to tutor for an artifact, and if it gets discard put it on top of your library. Argivian Resoration will allow you to put an artifact straight on the battlefield, you may even want to discard that artifact if you have this in hand. Rootwater Diver and Reconstruction both let your return artifacts to your hand too.
Black already has enough unconditional tutors that if you are running black, you shouldn't need gamble. Black also has some of the best recursion, especially for creatures, so if you do want to use gamble, you can probably always safely tutor for a creature and reanimate it later.
Buried Ruin will let you search for an artifact without fear, at the cost of a land.
Myr Retriver and Junk Diver will both let you tutor for artifacts without fear too. They can be recurred fairly easily, being creature.
Crucible of Worlds means you can search for whatever utility land or mana fixing you need and recast it form the graveyard if you have to. For enchantments, Crystal Chimes or Skull or Orm will both return it to you hand if its discarded.
Scarecrown works for artifact creature recursion.
There is also the ever useful of Library of Leng. This beauty of a card basically makes you impervious to discard effects. While they will still slow you down and get the card out of your hand, you can choose to put them on top of your library if you wish. It also works wonders with the wheel effects that most red decks already use, allowing you to keep the important cards in your hand, discard the rest, and draw a new hand.
In red, you can happily just tutor for Squee, Goblin Nabob. With Goblin Welder on the battlefield and any other artifact out or in hand, you can happily tutor for an artifact. Its the same if you have Trash for Treasure in hand. Recoup, Charmbreak Devil’s, Surreal Memoir, Past in Flames and Anarchist will all let you tutor for instants or sorcery's and potentially get them back if you discard them, though Charmbreaker Devils and Surreal Memoir are both random themselves!
Then there are cards that want to be in the graveyard anyway. Most popular of these are the incarnation cycle Valor, Wonder, Filth, Anger, Brawn. You might even want to get these in your graveyard! Searching for these with an empty hand is always nice, like a much cheaper red entomb!
There are also certain generals that don't care if you discard the card you a searching for. Generals like Sedris, the Traitor King or Adun Oakshield (though then you are probably better with the black tutors).
So there you have it. If you want to use Gamble effectively, make sure your deck has recursion for the type of card you are going to be searching for. Either that or cast it anyway, cross your fingers, and hope you get lucky! With enough recursion, you should be able to tutor for what you need most of the time, and you can still always cast it without recursion if you're in a pickle.