I was not so foolish, after all. The following line can be found on page 29 of the Manual:
NOTE: Ordnance weapons do not move in the game.
I have made a few experiments [:D]: Mortars, Infantry Guns (not the 15cm sIG 33) and AT Guns are able to move, even though larger guns have a greater chance to fail their movement check. Mortars (I only tried with a Russian 82mm) are also able to Double Time

. AA and ART don't move. I guess it depends on which guns are QSU types.
I do not remember (many years - at least 20 - have passed since I played ASL [:D]) how much time is required to emplace a Gun [&:].
Anyway, this possibility makes possible to have light AT/INF guns and mortars moving with infantry

.