I have one slightly drastic solution - but it involves effectively removing all the partitions. When I have been confronted with a similar situation I have simply zeroed out the first few kilobytes of the device using the terminal in order to remove the partition table. If you then unplug it and plug it back in again, none of the partitions can be mounted and gparted will see it as unallocated. You can then create a new partition table in Gparted from the Device menu, and then whatever partitions you need.
Assuming your SD card is sdb, this should do the trick:
Wait a few seconds until the terminal output tells you it has done enough - a MBR partition table is only 512 bytes long - and then ctrl-c to stop the process. Be very, very careful that you use the correct device identifier - you say sdb, but check this. One time that I did this, in a fit of absent-minded carelessness I typed sda instead of whatever it should have been and wiped out the partition table on my main multi-booting drive. The good news was that it was an opportunity for me to learn how to use testdisk in the real world.
sudo shred -vzn 0 /dev/sdb
One caveat - I have no experience of installing Android to a SD card and I wonder if the partition table may not be a simple mbr one. There might be GPT data at the end of the drive. In which case you might need to zero the whole drive.