Last I heard support for encrypted partitions in GParted was being developed, but no working code had been released. As such, while you can probably get away with moving the partition, or possibly even allocating it more space (provided you stick the more space onto the end and not the beginning) There is no way for GParted to resize the filesystem inside it, so the additional space won't be usable. You might be able to add the space on, and then mount the partition and resize the FS from there, but I'm not sure. I will test it when I get some free time and post back if I can make it work.
Your best bet is probably to decrypt it, shuffle your data around, and re-encrypt it.