gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

(or similar), add:

        position = gl_ModelViewMatrix * gl_Vertex;

In theory, it's possible to reconstruct the latter from gl_FragCoord,
gl_ProjectionMatrix and the viewport (which isn't available via a built-in
shader variable). In practice, it's typically simpler to add a separate
variable (vertex shader output, fragment shader input).

If you do want to reconstruct it from gl_FragCoord, gl_FragCoord.w is the
reciprocal of the interpolated value of gl_Position.w, which will normally
just be the negative of the unprojected z coordinate (the standard
perspective matrix has [0 0 -1 0] in the bottom row).

So, for the standard gluPerspective() matrix:

        nx = 2 * (gl_FragCoord.x - viewport_x) / viewport_width - 1;
        ny = 2 * (gl_FragCoord.y - viewport_y) / viewport_height - 1;
        nz = 1.0/gl_FragCoord.w;
        x = nz * nx * aspect / f;       // f = 1/tan(fovy/2)
        y = nz * ny / f;
        z = -nz;
        distance = length(vec3(x,y,z));

Note that zNear and zFar aren't relevant, as we're using gl_FragCoord.w
rather than gl_FragCoord.z.