Just thought I would post this, as I was having problems, and have solved the issue. Basically I couldn't resize a partition on a card I had previously used for debian, till I zeroed the card using:
sudo dd if=/dev/zero of=/dev/sdx
with sdx being the name of your card device. Be very careful with this command, get it wrong and you could trash your main drive.
Then just used gparted to resize and it worked.