Depending on the rest of your application, you might find the easiest path is to use the Gecko with the mbed.org tools. There already seem to be mbed Gecko development boards so the task is, to port the Rockblock-IridiumSBD Arduino library.
The benefit of using mbed with the Gecko is it looks like mbed have all of the libraries, including low-power control implemented.
The Rockblock-IridiumSBD Arduino library won't compile 'out of the packet' on mbed.
However, the Rockblock-IridiumSBD Arduino library appears to be a serial interface over a few wires. They recommend 'soft serial' which uses digitalWrite, under program control, and no deep access to Arduino peripherals. That should be relatively straightforward to port to mbed. Mbed has equivalent library calls for digitalWrite, so a first attempt might be a few global search and replace edits.
The alternative would be to use Gecko with an Open Source or commercial development system. However, mbed would provide a faster start, if you have good internet access and are not too paranoid about security, because the tools are 'cloud based', saving you time to set up the tools, and their is an mbed community who might help you over the initial hurdles.