In the full rules it is clear that a Baz may be fired at a gun:
C8.31:
HEAT may only be fired at a vehicle or Gun, or at Infantry/Cavalry receiving a wall/building/rubble/pillbox TEM.
In the SK Expansion Pack rules, 6.2 defines HEAT as
HEAT: (High Explosive, Anti-Tank): HEAT can only be used against Infantry in buildings or against Guns and vehicles. It has its own To Kill Table, and it attacks on the IFT column one lower than normal for HE. LATW show their FP on the counter. If available for other than SCW, it is listed on the back of the counter as "H".
Unfortunately, the SK BAZ rules do not direct you to this only saying you can target vehicles and infantry in buildings. Since a. BAZ fires HEAT, you can target guns directly.
The rule for use of BAZ against Guns is in the section prior to 4.4.1, 4.4 "Light Anti-Tank Weapons (LATW)". Per that rule, BAZ are SCW LATW, and "SCW may also be fired at Guns."
Because the rules on firing on Guns (especially HEAT) always confused me, I include some examples. Assume a squad and a Gun manned by a crew are in the same Open Ground hex. A unit fires its Baz. Because the squad is not in a building nor manning a Gun, it can not be affected. The base TH # is found on the back side of the Baz counter based on range. The DRM are all the usual TH modifiers, e.g. Hindrance, TEM (in this case +0 for Open Ground, but typically will be +2 for emplacement), target size, CX, Leadership, etc (see the To Hit Dice Roll Modifiers table).
If the hit is a critical hit, the Gun and manning crew are destroyed. If not, roll the effects DR (on the IFT, using eight FP for a Baz). If the original result is a KIA, the Gun and crew are destroyed. If the original result is a K/, the crew is Casualty Reduced (which will only make a difference if the Gun is manned by a squad or by SMC) and the Gun is malfunctioned.
If the result is not a KIA or a K/, the Gun has a Gunshield, and the shot came through the CA of the Gun, add two (Gunshield DRM) to the effects DR; otherwise, use the original effects DR. Use the IFT to determine the result based on the (possibly modified) effects DR. That result is applied to the crew. The Gun is not affected.
If the squad and the Gun with crew had both been in a stone building, both could be affected by the attack. Because of the Gun size DRM, the DRM for the two targets could be different. Depending on how they were different (i.e. which one was larger), one target might be hit and the other missed, or they could both be hit or both missed. Typically a Gun in a stone building will prefer the better TEM (+3) of that terrain over its emplacement TEM, but the Gun could choose the emplacement instead. Determining the effects is done as before. The squad, if hit, will use the same effects DR on the eight FP column of the IFT with no DRM.
JR