I've never used testdisk, but I quickly found this guide, which may help:
If you're lucky, you'll be able to recover your partitions. You may need to reinstall a boot loader in the disk's MBR to make the system bootable again. Depending on the options you selected in GParted, it's conceivable your filesystems are damaged, too, but if you only created a new partition table, they should be OK, just inaccessible because the system can't find them.
In the future, back up your partition tables. You can make a binary backup with dd ("dd if=/dev/sda of=sda-backup.mbr bs=512 count=1"), but that only backs up the primary partitions. To back up both primary and logical partitions, you can use sfdisk:
This assumes you're using MBR, which seems likely given that you had Windows on the disk. If you're using GPT, I recommend you use my GPT fdisk to back up the GPT data. It's the 'b' option on the main menu. Alternatively, you can do a dd copy of the first 34 blocks, assuming a standard partition table size. The dd copy will get the primary table, but not the backup, which is stored at the end of the disk. Still, restoring the primary will enable disk utilities to recreate the backup.
sfdisk -d /dev/sda > sda-backup.txt