I have a gun as a view entity (in first person perspective). How do I make it become illuminated by the lighting in the level?

Come on, nobody knows how to do this?

Have you read the magazine?

View entities are not getting illuminated by the lightning in the level, thats because they don't exist "in the level" but only "on screen".
You could illuminated them by reading the floor brightness (use c_trace) and adjust the ambient etc. values
Another way would be to not use a view entity but a world entity, make it passable and set its znear flag so it does not get clipped into walls.
Then adjust its position and rotation to the camera position rotation.

Use a "real" entity like this:

action my_gun() // attach this action to your gun
{
while (1)
{
vec_set (my.x, vector (20, 30, 35)); // play with these values
vec_rotate (my.x, player.pan);
vec_add (my.x, player.x);
my.pan = player.pan;
wait (1);
}
}