If you want to fresh install win 10 on the drive now you have copied everything off it, go through the process you already have and when you get to screen where you got the GPT error, delete all the partitions on the hdd and click next and win 10 should create 4 new partitions and continue the install.
What happened was win 10 recognised your motherboard has an UEFI bios and win 10 wants to use GPT format on the drive, and the error isn't written in a clear enough way to show you what it wants you to do. Your old drive may have been an update from win 7 and used MBR format on the drives. Win 8 & 10 use GPT, the main advantage being it can use hard drives bigger than 2tb, MBR cannot.
You wouldn't need a new copy of win 10 to install it on a new hdd in a PC that has already had win 10 on it. You have a digital entitlement to always install win 10 on that PC again. when you reach screen asking for licence now, click "I don't have a key" and win 10 will continue install and reactivate itself once finished. A record of your PC, possibly your email address, and your old licence are recorded on a Microsoft server. Win 10 checks that server to confirm you can install win 10 on that PC.
this guide might have helped: : http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html