Gods are illegal because on the most recent printing, it says in the lower left corner, "This card cannot be used in a Duel." Anyway, if I were to play the gods, here is how I would play them...in a way that they aren't uber broken.
Slifer the Sky Dragon
DIVINE
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Divine-Beast/Effect
A Tribute Summon of this card can only be conducted by offering 3 monsters on your side of the field as Tributes. The ATK and DEF of this card becomes the number of cards in your hand x 1000. When this card is Special Summoned, it is sent to the Graveyard during the End Phase. While this card is face-up on the field, it cannot be targeted by Spell cards, Trap cards, or effects of Effect monster cards that designate a target. When your opponent successfully Normal Summons or Flip Summons a monster with 2000 DEF or less, destroy that monster with this card's effect.
ATK/? DEF/?
GENERAL RULINGS ABOUT GODS:
If you decide to Tribute Set instead of Summon a god, you still have to Tribute 3 monsters on your side of the field.
Gods can still be destroyed by cards such as "Dark Hole", "Cyber Jar", etc. because they do not target. "Bottomless Trap Hole" can destroy it upon Summoning (assuming its ATK is 1500 or more) because it does not target.
You cannot activate "Divine Wrath" against any god's Tribute Summoning, because the text that requires three Tributes is a condition written into the card, not an effect.
If a god is Special Summoned in any way (not necessarily from the Graveyard), it is still sent to the Graveyard at the End Phase. For instance, if you somehow lower a god's level to 7, then use the effect of "Tribute Doll" to summon it, it will still be destroyed during the End Phase.
If a god is Special Summoned, but its effect is negated by a card such as "Skill Drain" at any time up to and including the End Phase, it is not destroyed, not even on the following End Phase, or any End Phase after that. It is not destroyed even if "Skill Drain" is removed from the field after the End Phase of the turn it was Special Summoned in.
DIVINE is not a legal declaration for "DNA Transplant", nor is Divine-Beast a legal declaration for "DNA Surgery".
You cannot declare "The Winged Dragon of Ra" or "Slifer the Sky Dragon" as targets for the effect of "Sangan" or "Witch of the Black Forest", since "?" is undefined.
CARD-SPECIFIC RULINGS:
"Slifer the Sky Dragon" cannot destroy Special Summoned monsters with his effect. This effect is an Ignition Effect and can be chained to. "Divine Wrath" can be activated against it.
If "Skill Drain" is on the field, the ATK and DEF of "Slifer the Sky Dragon" will be zero.
Obelisk the Tormentor
DIVINE
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Divine-Beast/Effect
A Tribute Summon of this card can only be conducted by offering 3 monsters on your side of the field as Tributes. When this card is Special Summoned, it is sent to the Graveyard during the End Phase. While this card is face-up on the field, it cannot be targeted by Spell cards, Trap cards, or effects of Effect monster cards that designate a target. Tribute 2 monsters on your side of the field to destroy all monsters on your opponent's side of the field.
ATK/4000 DEF/4000
Tributing 2 monsters for the effect of "Obelisk the Tormentor" is a cost.
You can activate "Divine Wrath" against the last effect of "Obelisk the Tormentor".
You can activate "My Body as a Shield" against the last effect of "Obelisk the Tormentor" to negate the effect, but "Obelisk the Tormentor" will not be destroyed.
The Winged Dragon of Ra
DIVINE
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Divine-Beast/Effect
A Tribute Summon of this card can only be conducted by offering 3 monsters on your side of the field as Tributes. The original ATK and DEF of this card become the sum of the ATKs and DEFs, respectively, of the Tributed monsters. When this card is Special Summoned, it is sent to the Graveyard during the End Phase. While this card is face-up on the field, it cannot be targeted by Spell cards, Trap cards, or effects of Effect monster cards that designate a target. When this card is successfully Special Summoned, you may activate one of the following effects:
-Pay 1000 Life Points to destroy and remove from play 1 monster on the field.
-Pay Life Points so that your Life Points become 1, and increase this card's ATK and DEF by the amount paid.
ATK/? DEF/?
If you Tribute Set "The Winged Dragon of Ra", its ATK and DEF are still the combined ATKs and DEFs of the Tributed monsters.
If "The Winged Dragon of Ra" is flipped face-down (such as with "Swords of Concealing Light"), its ATK and DEF will become zero. If it is flipped face-up later, its ATK and DEF will still be zero.
If "The Winged Dragon of Ra" is Special Summoned, its ATK and DEF are zero.
The effect that determines "The Winged Dragon of Ra"'s original ATK and DEF is a Continuous Effect and cannot be chained to.
You can activate "Divine Wrath" against the Special Summoning of "The Winged Dragon of Ra" only if your opponent chooses to activate one of "The Winged Dragon of Ra"'s effects.
If your opponent Special Summons "The Winged Dragon of Ra" and uses its effect to destroy and remove from play 1 monster on the field, you can chain "My Body as a Shield" to negate the effect, but "The Winged Dragon of Ra" will not be destroyed.
If "Skill Drain" is activated while "The Winged Dragon of Ra" is on the field, its ATK and DEF become zero. Its ATK and DEF will remain zero for the rest of the time it is on the field (even if "Skill Drain" leaves play later). See the rulings on "UFOroid Fighter".