The Artilery rules on page 55 acknowledge that "These units are quite complex" but are not entirely clear on how to resolve a lot of these scenarios.
What happens to the remaining gretchen if a gun is destroyed? Do they disappear, and why isn't that specifically stated? If they remain, I assume they cannot score since they are heavy support choices. If they stay, does that count as a kill point?
There's no special rule, so we'd default to the 1 unit = 1
KP. Thus, I suspect that killing the guns would give 0
KPs if the Gretchen survive. Finishing off the unit would give 1
KP. This is complicated by the fact that the Gretchen can fire at a different target from the gun, but in the absence of a rule naming them as a different unit, the 1 unit = 1
KP rule seems the best solution.
What happens if one grot is killed, does the remaining grot have to make a leadership test, as 50% of the squad got destroyed?
Yes. Page 55, header "Morale and Fall Back Moves" is pretty clear on this. The Grot would fall back (with the gun) and could test to rally next turn (provided he's still on the board).
What constitutes cover for the unit? I am assuming that each model (gun + 2 gretchen) all count cover for their type of unit, so if I had 50% of the unit (2 grot or grot + gun) in cover, I would get cover saves (gun of course being a vehicle using vehicle cover rules, the grot using infantry rules). I assume that I can basically leave the gun out in the open, and have two gretchen in cover, giving the entire unit a cover save.
That's my take as well. This seems analogous to
TMCs with Tyrant Guards: the vehicle may not benefit from area terrain, but as long as the infantry are in cover, 1/2 of the unit has cover, so the vehicle gets a save.
Are the grots (6 in the squad with 3 guns) all the same gretchin squad? It makes a big difference if losing 1 grot forces leadership on that gun, or if losing 2 grots forces leadership on the entire unit - that is potentially a huge deal, either way. It also makes a big difference when it comes to allocation.
Yes, they're one big squad. Losing one grot won't force a
LD test if you've bought 3 guns.
Basically a huge question is how does wound allocation work? When would I allocate? We start with ballistic skill, lets say with 15 lootas (orks is really all I know, so that's my reference) who are getting 3 shots, so that's 45 shots, with BS 2 they should successfully fire 15. It wouldn't make sense to allocate BS shots, so lets say we now roll and determine that 10 hit the guns, and 5 hit the crew. If I allocate these, am I allocating 15 shots, or do I allocate the gun and crew shots separately? Would I have to do 4/3/3 hits on the guns and 2/2/1 on the gretchin? Or would I straight up do 5/5/5 and allocate them however I like? (ie. 5 gretchin hits on one, 5 gun hits on other, 5 gun hits on last?). If all the gretchin are the same squad, and we only then allocate the gun hits, that'd make sense and be a whole lot easier, but I don't know that that is how this would work.
Well, the artillery rules already give you the algorithm for randomizing hits among the crew versus the gun. After that, you allocate successful wounds (not hits) on the crew just like any other infantry unit. If you look at page 25, "Taking Saving Throws", it's pretty clear that you make a separate roll for each gun crew, since even though every grot has identical stats and wargear, they are not "identical in gaming terms." You do NOT make a separate roll for each grot, unless there's some option to kit out their wargear.
Similarly, you would analogize to the squadron rules for vehicles to assigning hits: "the player controlling the [vehicle] squadron allocates the glancing and penetrating hits to squadron members as he would allocate wounds to members of a normal unit." (p 64) Once again, the rules stating that all glances and pens kill artillery mean it's impossible to pull any sort of wound allocation tricks unless the guns are in cover. If the guns
are in cover, I'm not 100% sure what you should do. Are the guns "identical in game terms?" I believe that the grots are not since they tend different guns, but it's not clear if that relationship goes both ways. (Are three Zapp Guns "different" because they have different grots tending them?) I'm not sure how to resolve this one...