How do I set a manual gas price for a contract calling a function? And is nonce management implicit?

I have created a contract object with ethers.Contract giving it ABI and address. My necessity is to set a gas price that is constant so that I can do the transaction with lower fees. The problem is I cant find a way to set my gas price.

var provider = new ethers.providers.InfuraProvider('ropsten');
const privateKey1 = Buffer.from(process.env.PRIVATE_KEY_1,'hex')
var wallet = new ethers.Wallet(privateKey1,provider)
wallet = wallet.connect(provider)
......
var contract = await new ethers.Contract(address, originStampABI, wallet);
for(let hash of hashes){

	var sendTransactionPromise =  await contract.submitHash(hash)
        sendTransactionPromise.gasPrice = 20000000000
	sendTransactionPromise.gasLimit = gasLimit
	console.log(sendTransactionPromise)
	/* sendTransactionPromise.then(function(tx) {
		console.log(tx);
}); */
}
}

I could only find the way to set the gas price by calling the gas price below the smart contract function call which seems to not work and give default gas price of 1GWEI in etherscan. I am using infura as the provider Also is nonce management implicit in ethers.js. I have not specified any nonce but it seems to be working

[yuetloo]

@sssubik , by default, if you don’t override the nonce, ether.js will populate that for you by querying the transaction count from the network. But, if you want to use loop, try the experimental package, NonceManager, it increments the nonce for you.

[yuetloo]

@sssubik , use the override parameter, see doc here and overrides properties here

    var overrides = {
        gasPrice: 20000000000
    };

    var sendTransactionPromise =  await contract.submitHash(hash, overrides)