It is not possible to "add" space from one hard drive to any partition located in another hard drive.
It is possible to re-arrange partitions in a hard drive to make unallocated space next to a partition, so that we are able to grow that partition.
With a new hard drive, it is possible to copy there amount of our data. This manner we could perhaps to shrink a partition and gain the unallocated space I mention above.
It would be possible to copy (move) partitions from a hard drive to another. However, this could trouble somehow the situation, especially in the case you have software installed in the partition you move.
You can find advice on partition modification, in the Documentation page
http://gparted.org/documentation.php
in particular in the "Help manual" and "Man page".
There are some older help files too, but I'm afraid they are outdated in some degree, because GParted did change largely since then.
The main purpose of GParted is to modify existing partition, to set some attributes on these partitions (like "bootable", "LBA" etc), and delete/create partitions and partition tables.
As far as I know, the only way to "merge" the space of 2 or more hard drives is the RAID configuration. However, this needs a rather high level of knowledge and expertise in information technology and operating systems, and needs to reinstall the operating system. Furthermore, special hardware is needed, too (a special RAID controller).
(moved to the Live Media section)
*** It is highly recommended to backup any important files before doing resize/move operations. ***