How can I launch a GUI application in a fullscreen window without a complete desktop environment?

Is it possible to open a GUI application in a single fullscreen window from the terminal without all the other aspects of a desktop environment?

For example, Firefox.

Yes.

You haven't provided any details of your GUI application; and importantly I think what stacks it uses.

Is it motif? simple xorg?, using GTK2/3? Qt5? etc as most applications require specific stacks that need to run - but if it's a simple X11 GUI app that doesn't use stacks I believe it's a yes. However very few GUI apps remain today that don't use GUI libraries/toolkits.

I've opened GUI apps on unix/sun-OS boxes where the size was defined in the command that opened the window, and full-screen was possible, however they weren't modern apps & didn't use modern toolkits. Many of those programs also ran in Debian & Ubuntu GNU/Linux the same way, but it's been decade+ since I've wanted to run them as they're outdated (with many no longer available, as they were re-written/replaced and it's simpler being able to use a WM than controlling everything from command line).

You however gave no specifics; but it is possible for some applications (just not worth it in my experience; we no longer run machines with that limited resources).