Hey Brian,
the Onefinity CNC machines are stand-alone machines, which means, all you need is a USB stick with your g-code file on it. You stick it into the machine, open the file, and let the machine execute the g-code toolpath.
Unfortunately since the underlying operating system on the Onefinity is outdated and does not have exFAT filesystem support installed, which is usually the formatting of new USB sticks greater 32 GB nowadays, usually you need to reformat your USB stick with NTFS or FAT32 or use a smaller one that comes with FAT32 formatted. USB sticks formatted with ext2/ext3/ext4 are also supported.
exFat would be supported natively with Linux kernel 5.4 but I have no idea if or when Onefinity will switch to a newer version of the underlying OS.
Although there exist g-code editors, you only need a plain text editor to write g-code. After saving the file, you rename the file extension from .txt to .ngc, and the machine will recognize and accept the file.
In ancient times all g-code was written by hand. Nowadays you usually use CAD/CAM program to export a toolpath as g-code file.