[Ethereum] Coinbase and Kraken Transfer Costs

• When i transferred varying amounts of ETH from GDAX to Kucoin, the Gwei seems to be automatically set at 100+ (110 to 122). My understanding is that that is a pretty high Gwei and is not necessary. I can't seem to figure out how to adjust the Gwei value though. Is there a market exchange specific setting to adjust Gwei for xfering ETH?

The "GWei" is actually called gas Price. It is the amount of ether you accept to pay for each gas consumed. The higher it is, the faster your transaction will be mined (to explain it in a simple way). But you can set this value as you want (Using Mist, Myetherwallet, etc).

• The "Gas Used" amount also seems to remain constant for any amount of ETH xfer i tried (at 21,000). Is this a value that i can adjust? Or is it always set like this?

You cannot adjust it. It is established by the ethereum virtual machine. 21k is the very minimum for a transaction (basically, a simple ether transer will always be 21k gas). As a user, you want this value to be as small as possible to not pay to much.

• Is this a not-so-uncommon problem that has to do with Kucoin (like they somehow "lost" the incoming transfer or something)? How would you resolve such issues?

Need more details about what you are trying to achieve.

• Is it always a better idea to transfer a large amount of ETH / BTC at once, instead of small amounts in multiple transactions?

(My answer is for ethereum only, bitcoin works in a (very?) different way) Yes. Because one tx costs AT LEAST 21k gas. So if you transfer 10 eth in 10 tx (1 by 1), you will pay 21k * 10 + (additionnal contract gas needed for processing). If you do it once, you pay 21k * 1 + (additionnal gas needed by the contract).

• Also, the transfer fee doesn't depend on the amount you transfer, is that correct?

Yes. The tx fees depend on gas, not on amount of token/eth transferred

• What is gas, how does it work?

A transaction has 4 properties related to gas:

1. gasLimit: The maximum amount of gas you accept to use. This is an internal security to avoid infinite loops and then potentialy infinite costs
2. gasCost: The actual amount of gas used. If it used less gas than the gas limit value, you get charged only for gas used.
3. gasPrice: The value in ether for a unit of gas.
4. txFee: This is the actual fees you paid for the tx, it is: gasPrice * (gasCost + 21 000)

Both transactions used the same amount of gas (21000 - minimum for a transaction). The second transaction paid a higher price for the gas (30 Gwei vs 21 Gwei). Not sure about how MyEtherWallet works, but the educated guess is that getting transactions done quickly required a higher gas price at the time of transaction #2. Usually you can override the gas price, so if you're not in a rush, you can dial that down to as low as 1 Gwei and lower your transaction fee accordingly.