Is there a way to run a GUI application (GNURadio in this case) from [a virtual console]?
I've replaced "command line" here with VC (sometimes VT, virtual terminal), because that's what you are referring to. You can of course start a GUI application from the command line in a terminal emulator on a running GUI desktop. The nomenclature here can be confusing (non GUI "virtual terminal" vs. GUI "terminal emulator"; the former is still considered virtual because historically, a real terminal was an actual piece of hardware, whereas your screen and keyboard are interfaces to a virtual software version).
Anyway, the general answer here is no, although there are exceptions. This is not one of them, since it depends on GTK, which is a widget library dependent on the X windows stack.
a full desktop environment to conserve resources
A "full desktop environment" may be ambiguous since the technical definition of desktop environment in the context of the X stack is not something that is required to run a GUI app. All you need is the Xorg server running and the GTK libraries installed. This is probably less than 100 MB of physical RAM.
Note that like everything else desktops don't consume much (or any) extra CPU except to the extent you are actively using them (moving around, opening and closing windows, etc).
I have tried the following:
startx gnuradio-companion ... However I get a fatal error
I don't think that error is necessarily the problem. You should try and get this working from the command line in a terminal emulator on a normal desktop then work backward from there if you want to do it directly via