How do I recover a data partition: gparted interrupted while extending it ...?

[ptyerman]

Ouch! Avoid any more work using Gparted or any other partitioning software as it may make things worse. Have you tried using Testdisk to look for the lost partition? If it finds it there is a very good chance it would recover the original partition. Use the extended search option, it will yield better results.
I've recovered partitions before using Testdisk, it's extremely good at what it does.