NFT_PRESALE.sol Why is this GPL? Where Did You Get The GPL Code?

Hello Hashlips, I'm a big fan of your code. I want to use NFT_PRESALE.sol for a dapp I'm building but I don't want my dapp to be required to bear the GPL license. Why is this solidity contract GPL? I'm assuming you're making it GPL because some of the code in it comes from a separate GPL project. If you could tell me where the GPL code comes from I could take that out or alter it for my project.