[Ethereum] How to get the historical Gas (Gwei) Price

EDIT: with EIP-1559 (shipped in London fork in July 2021) this is not true anymore. I'm changing this answer to community wiki so that anyone can fix it (I don't really have time, sorry)

As explained in the Ethereum github's wiki the gas price is set by miners and the only way to guess the acceptable value is to look at the last block gas price. Then you look at the lowest and highest prices on the block and next block price should fit close to these values. Looking at more blocks in the past could give you a trend to guess if the price raises or goes down. But there is also a part of random, at least in Geth, so you can't exactly predict the gas price but only be close to the future block price. I think you can't be more accurate than geth gas price oracle.

There is no minimum for gas price

You can set a transaction with 10wei as gas price if you want.

The problem is most miners won't include transactions below it. You can see here http://ethgasstation.info/ the minimun gwei each miners pool ask for. It seems right now the minimun is 1gwei and it will take 1.5 minutes.

However with tools like MyEtherWallet or Metamask the minimun is indeed 2gwei because actually it is "the cheapest" miners accept.

But in the future we might see this reduced again but for this more miners will have to accept transactions below it.

If you want to set manually your gas price I advice you to use a client like geth

eth.sendTransaction({
 from: '0x036a03fc47084741f83938296a1c8ef67f6e34fa',
 to: '0xa8ade7feab1ece71446bed25fa0cf6745c19c3d5', 
 value: web3.toWei(1, "ether"),
 gas: 100000,
 gasPrice: /* gasPrice in wei */ 
})

Remember one gwei is equal to 1billion wei