I have USDT worth $100 in myetherwallet but i don't have enought ETH for gas and now the gas is costing approx between $5 to $15 as it's dynamic but if i have to transfer the ETH from my CEX wallet it's costing me around $10 to $16,
The best way is to wait for the gas fee a bit cheap. Your coin is USDT, you don't need to worry about the price, it is a stable coin. What you need is to buy ETH at a cheap price. Because we seem to have a decline phase in the next few days, it is your chance to buy ETH. Then, you can send it to your MEW. or if you need urgent fresh money, you just force to send ETH to your MEW now. We don't know when the gas to be cheap enough.
This is great advice, honestly for now that ETH is not at its maximum price, you should buy it and send it to your MEW, however it is the best advice so far, 15 or 16usd may sound like a high fee, but until If the rates do not drop, it is not possible that you can make new turns, if the price of ETH rises as BTC rises, it will be more difficult to pay the corresponding gas, although currently I prefer to use Metamask instead of mew, it seems to me that it is safe and that in the same way you can make your tokens available or in this case the usdt.