Not aware of any obvious reason why it wouldn’t work. If there are specific concerns, please state them; likely they would be quirks which could be worked around.
If you want to make a gateway out of an existing pi installation, you should probably follow the generic instructions for installing gateway software on a pi and put in whatever reset GPIO RAK is using. The RAK board isn’t really anything special, it’s mostly just an SX1301 or SX1308 chip on an SPI bus as available from a variety of manufacturers, so you can use the software directly from Semtech’s repositories. (Er, well, if this incorporates RAK’s usual design mistake you’ll have to drop the SPI clock to 1 MHz or so.)
Using RAK’s SD card image would require that you reinstall whatever components you were previously running.
Best bet is to get another SD card for these experiments anyway, make a copy of your setup (yes, that’s a little tricky unless you can use
dd on another machine to do the copying) and work on that.