The "hot glue gun" is just a tiny part, namely the extruder hot end. Add to that a precision (computer-controlled) feed mechanism for the "glue", temperature regulation to work optimally with different feed rates and "glue" types, and a precision, high-speed, XYZ positioning mechanism for that "glue gun" (and optionally, additional "glue guns" so you can switch materials in mid print), together with a computer and firmware to drive all, and you're approaching what even the lowest-end consumer 3D printer does.
"Glorified"? Yes, and it is glorious. Perfect? Of course not, not any more than a cheap consumer Epson or Brother printer is compared to an Espresso Book Machine.