Unfortunately as class413 said GParted doesn't change partition table types, it removes the old partition table and writes a new partition table. That is why the Create Partition Table dialog says the following in large bold letters:
WARNING: This will ERASE ALL DATA on the ENTIRE DISK
Just to update what class413 said about the secondary GPT being available. GParted is very likely to have erased both the primary GPT at the start and the secondary GPT at the end of the disk. It writes 512K of zero to the start and end (last whole 512K) of the disk to ensure that old signatures are erased before new partition tables, or file systems within partitions, are written to prevent all sorts of tools getting confused by detecting multiple different signatures.
As class413 said, testdisk was your best option to re-discover the original partition boundaries.
After that you will have to start manually guessing partition boundaries, using GPT and file system tools and possibly even hex editors looking for the file systems. Alternatively you might be able to find a profession service which can recover the data for you. I have never tried one or even looked for one.