I can't comment on the Synergies of Gravity all that well, because I don't frequent with Mono/Duo decks; it's because of all the time I spend with rainbow decks. Forgive me.
However, while playing around with said decks, I have noticed a few cards from the other elements that do well with the likes of the Heavyweight Element of the game. Allow me to start with the Elements I voted in the poll:
AETHER
Not surprising anyone, Aether is my first choice for Gravity. Aether has FOUR cards that work alongside Gravity:
- Quintessence - Invincible creatures that simply dominate on the field of battle (Sapphire Charger/Otyugh/Titan)
- Parallel Universe - Aether laughs at the six-creature-card limit (Sapphire Charger/Titan/Armagio/Colossal Dragon)
- Dimensional Shield - Bypass enemy defenses while stopping them from getting past yours (Sapphire Charger/Catapult)
- Fractal - See Parallel Universe (Sapphire Charger/Armagio/Graviton Mercenary)
Aether turns the already devastating Gravity Element into a war machine in a way no other Element can; whether you need an offensive boost, or a defensive save, Gravity's got Aether's support. It's not cheap to do so, but once the cards are played, you'd better hope the inevitable isn't coming after you.
ENTROPY
The public vote for Entrophy so far has been about the combination of Discord + Black Hole, and it's a bit obvious to see why that is. However, that's not why I voted for this Element; I've got two words for you:
Chaos Power
Sure, Blessing can do the same thing (and sometimes better), but the immediate availability and readiness of Chaos Power gives it a distinct advantage. Being able to play this off your Mark alone means being able to play one or two of them quickly on one or two creatures within the first few turns.
Chargers, Titans, and Otyughs will easily benefit from the swift moves of the chaotic strengths, suddenly become more dangerous than they already are; heck, any creature benefits from the effects of Chaos Power (though some more than others, as previously mentioned).
I could make an argument for Maxwell's Demon + Gravity Pull, but I've never tried that out. If someone has, feel free to comment on it; I'm a tad curious about it myself.
FIRE
Fire's massive CC arsenal is just begging to be called upon, and with good reason; it turns gravitational force into a weapon of mass destruction. Rage Potions are useful with several Gravity creatures, while Rain of Fire and Fire Bolt turn battle-hardened warriors into marshmallows. The health of Armagios and Colossal Dragons shine through as the abilities of Sapphire Chargers and Otyughs leave a lasting and burning impression on any opponent. 8) Deflagration prevents any chance the enemy can stall you while Fire Shield makes you wish you had just stayed home. Fire keeps Gravity warm at night while Gravity keeps the Fire from burning out.
LIFE
The poll says Life isn't a good option for Gravity, and, to some extent, it's warranted. However, if you've ever been on the receiving end of a Sapphire Charger on drugs, you'd understand very quickly that with Life comes PAIN!
Adrenaline is a nasty card to be paired with Gravity; one card turns a four damage-dealing Charger into a nine damage-dealing powerhouse. And don't think shields are gonna make it hurt any less either; once the Adrenaline kicks in, it's gonna be a bumpy ride. All of Gravity's creatures benefit from an Adrenaline rush... well... maybe not Armagios, since they're more meat shield than meat cleaver.
Mitosis is another good option, since you can spawn an infinite army of Mercenaries/Guards, Chargers, Armagios (near-infinite defense, anyone?), and, if you're really spiteful, Titans. It's bad enough having to deal with Gravity throwing it's weight at you once; now imagine having to dealing with it multiple times.
I could make a few more combinations, but those were the four I voted for, as I feel they're the best for Gravity. Will I make a vouche for the other Elements? Maybe. But that's for another time.